I've said something similar to what I'm about to say on previous threads related to this topic. As much as I typically agree with Q36 and think he is one of the best assets of this board, I wholeheartedly disagree.
I'm four years out of grad school now, and here are two lessons that I think every current candidate should take to heart:
1. your employer doesn't give a shit about treating you "ethically." They care about not getting sued/breaking the law and about profit-maximization (or whatever the objective for academic institutions is assumed to be). If it suits his purposes, your employer will renege on an offer or a contract without a second thought for what that does to your life. I've seen two guys laid off while in the middle of their 6-year tenure track contract, and not for any sort of ethics breach or other offenses (other than perhaps not being as productive as one would hope). No consequences to the school. Maybe there was fine print somewhere, similar to the "subject to funding and approval" line that is in most job ads. Those are called weasel words for a good reason - your employer is a weasel.
2. The further into your career you get, the more unimportant are your adviser's opinion and any sort of reputation effects that might arise from reneging on an accepted offer (unless the only reason you have achieved any modicum of success is because your superstar adviser really helped you out). This is because of two things: a. Assuming that you are any good, you will quickly branch out, create your own network, and move well beyond your adviser's reach, and
b. People forget things that happened more than 1 year ago.
In all cases, you should do what is best for you. Your employer will certainly do what is best for him. But you also must remember that this means you can't trust even a verbal offer. Suppose you renege on school A to take school B's verbal offer, but then school B backs out. Where does that leave you? Even if you have it in writing, there are probably weasel words that can let them renege. What are you going to do, sue?