sadly, i don't have much time to say anything real about these posts, both of which are smart and elegantly argued. i am and probably will remain committed to the idea that third- or multi-partyism is a no-go in the united states without major constitutional reforms. we are not going to end up with an electorally viable third party just by changing ballot eligibility requirements. as henry points out (without citing to the examples he speaks about, though), there is some flexibility in duverger's law ("winner-take-all congressional elections in single member districts produce two-party systems" is a decent paraphrase) -- but america has hardly shown itself to be an exception. so yes. while in some countries perhaps there is a chance for more ideologically representative party systems, we're not going to see anything like that here.
which, i think, is a shame. i disagree with tim burke that your choices are limited to [voting for nice guys in attractive suits who seem generally OK but whose platforms leave gaping holes in your preferences] versus [voting for one of a trillion fragmentary ideological parties]. furthermore, i think it's worth noting that institutional forms DO change both political thought (public opinion, not academic philosophy, i mean) and policy outcomes.