Unless one resides in a state (or Congressional district in those states which split the electoral votes) that is very safely in the Bush column, then and only then, can one safely give a boost to ones third part of choice cannidate. The same logic applies in Congressional and lower races. Since the Incumbent wins most of those anyway, the danger in voting third party is even less, but check to be sure in your district.
Soon the Democrats will implode. Exactly what "soon" is, I can't say. But when they do, you can be assured that new party, or new parties, will be formed. I could see the less rabid Dems joining with the McCain wing of the Republicans and the more conservative Repulicans becoming something else, hopefully something which holds the basic principals of the Constitution in higher regard than either major party does today. Those basic principals, aside from protection of individual liberty and rights, include separation of powers, federalism, and most importantly limited government.
I'll tell you what: I would fully support a shift to an approval voting based system. Basically, you can vote for as many candidates as you want, (though at most one vote per candidate of course :)) This lets people vote for third party candidates without fear of serving as a spoiler.