I think that many underestimate the intelligence of voters, at least in terms of voters recognizing that they've been screwed by the repubs, deep and repeatedly.
While they may not trust Dems entirely, and often see Obama as an unknown quantity, they know and will become acutely aware that voting for McCain is basically voting for McSame- that a McCain Admin won't be enough different from a Bush Admin to effectively take a more positive direction. It can't be. McCain's just the front man- the power behind the throne won't change. Few will make the mistake of thinking that McCain represents someone other than the "true Bush constituency"- he wouldn't be the nominee if he didn't.
Running against Hillary just puts the well conditioned Hillary-haters firmly in the McCain camp, and lets him avoid the "McSame" label as he represents that Hillary is largely "McSame", as well... whether that's really true or not. Dems need to differentiate themselves strongly from the other side, and that's a lot tougher for Hillary...
These early polls are meaningless, anyway, considering that the Dem nomination is still up for grabs. Only some sort of obvious subterfuge in the process will prevent Dems from rallying 'round whoever that choice may be. With a little luck, that won't happen, and John McCain will simply become a sacrifice...