Here's an important observation from Mark Kleiman:
Here's a cheerful finding from the Pew poll: neither Obama's unfavorables among Clinton voters (now 30%) nor Clinton's unfavorables among Obama voters (now 31%) have been rising noticeably . So it looks as if (so far) the bitterness of the battle is largely restricted to the political junkies who read and write blogs.
This jibes with my experience of talking to not-so-political folks. It also, I think, explains a lot of the volatility in the race. You have a large number of people who like both candidates and, as a consequence, can very easily be swayed from one to the other by relatively minor turns of events.