This is an interesting question for a lot of conservatives. Even Ann Coulter, firebrand Republican that she is, declared she'd campaign for Clinton before voting for McCain.
Dafyyd ab Hugh, proprietor of Big Lizards, makes the case for holding your nose and voting McCain if he's the nominee. Perhaps he is being more realistic than anyone else in approaching this. Perhaps he is thinking of the worst case scenario if Republicans should lose this fall. Take it as you will, but I highly recommend reading it.
A short excerpt? Okay:
If John McCain is nominated and loses, the next Republican nominee in 2012 will likely be more liberal, not less. There is no "great conservative hope" waiting in the wings to sweep to victory four years later, as in 1980 (or to sweep to catastrophic defeat four years later, as in 1964).
In the meantime, we'll have eight years of President Obama or President Hillary... in which so much of what we consider America is destroyed (politically or literally) that what is left is simply not a country we can imagine. If we think that there is no difference between McCain and either of those two bozos, then we have let passion make a fool of us.