Depends on the Greens. Obviously for at least half of the would be Nader supporters all it takes is having some prominent liberals (Gloria Steinem, Sierra Club leadership, et al) to announce their staunch support of the candidate, the impending doom of pro-choice and death of the environment at Republican hands.
For me personally. It would take a Democrat who comes out and _explicitly_ says they are in support of the formation of new political parties, they would have to support radical electoral change (IRV, public financing, prop. rep, etc) and democratization of media (open up micropower licenses to a 1st adjacency standard, reregulate ownership, re-reserve the left of the dial for non-profit, and crack down on local public affairs requirements for commercial broadcasters). If they did all that, a cabinet post for a prominent Green would be icing on the cake.
The key though is that I have to believe that them getting elected will directly help the Green party as a party. I want to change the system that locks us into two parties because it is structurally flawed. If abandoning the party for one super-progressive means the Greens fall apart organizationally, become complacent, then in 4 or 8 years it will all have been for nothing and we're back to square one. The rules of the game have to change and that's the number one most important political issue of our day. I refuse to live from election to election making only short term algorithmically greedy (who is the best candidate that can win NOW) choices.
But there are other Greens who just want to make the Dems more progressive with a little pressure. Those Greens will probably break ranks more easily than I will.