If you're going to vote Green this year, I think you would be making a mistake. Principles are important, but the Democrats are fielding about the best candidate they've had in a long time who is able to make some of the basic Democratic ideas seem palatable to moderates and even people concerned about "Big Government".
If the left is going to Kerry-ize Obama and start saying "At least he's not McCain!" then they will help elect McCain, and quite frankly, remove McCain's biggest weakness--that nobody cares about his campaign--and place it on Obama as well.
I don't really believe everyone here who says they won't vote for Obama if this or that happens, but I do have concern for them over their lack of enthusiasm and how it influences others.
The left doesn't have guts. If they really were serious about their core issues, they would have dumped Kerry for being complicit in the war (and effectively advocating John McCain's Surge policy as a candidate, if you recall), and they wouldn't have elected a bunch of Democrats to Congress who failed to end the war.
In the past, the Democrats have been willing to give up anything for the prospect of winning over a dreaded Republican...that is, until now (seemingly). ...Why change now? This is your best shot.