Denny Burk has written a fine piece criticizing this decision. I won't repeat his arguments here. I think H. Scott Baldwin and others have demonstrated that the best rendering of the Greek in 1 Timothy 2:12 is "exercise authority." I'm no professional, but from all I've read I think "assume authority" is a mistake on translation grounds. At the very least, it's odd that the NIV thinks the meaning of authentein has gotten less clear from 1984 to 2010, when the scholarship that's taken place in the last 25 years suggests the NIV got it right back then.
But I don't want to talk about Greek. I want to talk about English and what the word "assume" means. Many complementarians object to the new NIV translation, not only because egalitarians have been pushing for this rendering (as Burk points out), but because "assume authority" communicates something different than "exercise authority." For their part, the Committee on Bible Translation (the group of scholars responsible for the NIV) insist that "assume authority" was chosen precisely because it does not side with either egalitarians or complementarians. Craig Blomberg and Doug Moo, for example, maintain that the NIV rendering does not tip the scales one way or the other. Their goal was to stay neutral and bow to no theological agenda.
Blomberg and Moo are among evangelicalism's best scholars (and complementarians too). I use one of their books almost every week it seems. They deserve our respect and trust. We should take them at their word: CBT was not trying to play favorites in the debate over gender roles.
But acknowledging this does not mean we can't still disagree with the CBT's decision on 1 Timothy 2:12. As I said earlier, quite apart from the Greek, I think their English rendering does not do what the committee thinks it does. The new NIV obviously moves away from a complementarian-friendly translation of the text (as per the 1984 edition). The 2011 NIV may strive for neutrality, but on this issue it's definitely migrated in a certain direction.
More to the point, look at these two quotes which defend the CBT approach to authentein. The first from is Blomberg:
I can tell you authoritatively that we did NOT choose this rendering to tip the scales one way or the other. Whether you are a complementarian or an egalitarian, you have some view of what Paul thinks women should not do here, in terms of exercising authority. When they violate that, whatever it is, they inappropriately assume authority. That's all we were saying.
And here is Moo:
Moo wrote, "[T]he translators believed that 'assume authority' could be taken in either direction. We often use this phrase in a neutral way (e.g., 'When will the new President assume authority'?). ... [I]t is our intent to provide a translation that is faithful to the text, bowing to no particular theological agenda.
The argument is that "assume authority" is neutral because it can be read in two different ways. Indeed, the first two definitions for "assume (used with object)" in this online dictionary are: "to take for granted or without proof; suppose; postulate; posit" and "to take upon oneself; undertake." The third definition is closely related to the second: "to take over the duties or responsibilities of." So "assume authority" can mean "supposing you have authority (that you don't have)" or "taking on the duties or responsibilities of authority." The first definition in the last sentence sounds more like "usurp," which strengthens the egalitarians case. The second definition sounds more like "exercise authority," which helps the complementarian side (and is truer to the Greek in my estimation, the KJV rendering notwithstanding). The CBT figures "assume authority" plays it safe in the middle; neither side can claim it for their own.
The problem, I fear, is that most English speakers will hear "assume authority" in the "usurp" sense, not in the "exercise" sense. Moo gives the example: "When will the new President assume authority?" He argues this shows "assume authority" can be quite neutral and need not imply a wrongful grabbing of authority, which is what egalitarians hope the verse says, because then Paul is forbidding the illegitimate appropriation of authority not the exercise of authority itself.
But Moo's example may not be the most germane. First off, it's a question. And second, it looks to the future. In asking "when will the President assume authority?" we know by context that a neutral view of "assume" is in view. No one asks, "When will the President begin to assume authority he doesn't have?" By asking a question that looks to the future we know "assume" means something like "take over the duties or responsibilities of." But when we tell someone "do not assume..." most hearers are already thinking of a pejorative sense of the word. Unless we are talking about presidents or generals coming to power, we don't normally use "assume authority" in a neutral sense. I would argue most of us hear "assume authority" as "presuming to have authority (we don't)" or "taking authority for oneself." In both cases, the problem is not with the authority per se, but with the means of obtaining it.
This is the problem with the NIV rendering of 1 Timothy 2:12. "Assume authority" will be heard, despite the intentions of the CBT, as authority inappropriately gotten. People will not think of "assuming the responsibilities of the office" as in Moo's example. Why would Paul prohibit persons from assuming the responsibilities of their office (ala the President) anyway? No, to "assume authority" implies the authority was not simply exercised but was taken wrongfully.
I believe we see a tacit admission of this point in the Blomberg quotation above. "Exercising authority" is used broadly in the second sentence; then "assume authority" is used negatively in the third sentence with the adverb "inappropriately." If "exercise authority" is the best translation, then authority is the problem. If "assume authority" is the best rendering, we are dealing with the inappropriate assumption of authority. This change's Paul's prohibition considerably. The CBT cannot have it both ways. In the Translator's Notes the CBT states, "The exercise of authority that Paul was forbidding was one that women inappropriately assumed, but whether that referred to all forms of authority over men in church or only certain forms in certain contexts is up to the individual interpreter to decide." But is the interpretation really up for debate if it has already been established that the problem in Ephesus was with authority "women inappropriately assumed"? Does this not suggest Paul's command is about the assumption of authority, not the mere exercising of it? And even if "assume" leaves the door open for a more neutral interpretation, is this really how most English speakers will read the text?
Taking into account the ear of English readers--which is the NIV's translation philosophy--I have to conclude that the NIV rendering of 1 Timothy 2:12 is not neutral. At best, "assume" still implies taking authority. At worst (and more likely), the NIV makes it sound like Paul is against the inappropriate assumption of authority, not women-over-men authority in general. And this understanding is precisely what egalitarians have been arguing for and what, according to recent scholarship, the usage of authentein in Greek literature argues against.