Assuming he did run a third-party candidacy, it's hard to say which party he would hurt more--it would depend in part on who the major party nominees were. I can see some Republicans buying his claim to be a return to the old values of the Goldwater era, some Democrats voting for his foreign policy ideas, and some young people supporting him for his "authenticity"--it's hard to say which major nominee he would hurt more.
I'm not opposed to third-party candidates in principle--people ought to be able to vote for whomever they choose--but I would prefer any significant third-party candidates in 2008 to be on the left.
I know of a very hard core liberal who is promoting Ron Paul, so I know he will pull some Dem votes. I’m actually surprised this liberal isn’t supporting Kucinich.