Actually, could somone explain why other countries with first-past-the-post systems, like Britain or france, manage to have more than two parties?
Mostly a combination of elections being more local, political campaign style and laws, the nature of the parties, etc.
And to expand on what MSH said;
In Britain at least till 2010, the big two had fairly strong party discipline and ran on party recognition while the third/smaller parties were either regional or had weak enough party discipline that a South West Lib Dem would be a conservative alternative to the local Tory while a Northern Lib Dem would be running to the left of Labour. Labour and Conservative candidates had less flexibility in this area while getting tagged by the high profile party leadership.
Arguably Nick Clegg destroyed this by running the charismatic leader role as a national campaign. Combine that with the coalition and arguably the Lib Dems will never get to run that play again.
In the US both of the big parties are Lib Dem style coalitions, so they already run the chameleon role in each constituency and leave no room for a local interest or flexible-interest national party to fill in such gaps.
FPTP messes up election results and hurts smaller parties, sure, but I don't think it's even mostly to blame for their lack in US politics.
Uh, I thought that there were a lot of reports of people not being put on the ballot beyond the Big Two? o-o
It can be tricky in places, but usually that's more about being frozen out of debates and things like that. Usually you need to either pay a fee or gather a given number of signatures to run for office. Even most joke candidates can pull that off. Getting into debates usually means you are over a certain polling threshold, that threshold usually being high enough that no third party candidate will meet it. Even 5% is a deal breaker, but I'd bet a 10% 'spoiler' candidate would see thresholds set at 15%.