It, in fact, is not what keeps the two-party system in place. What keeps that in place is winner-take-all voting. Unless three parties are equidistant politically, two of them are liable to steal votes from each other, thereby making the remaining party more likely to win. Also, even if they are equidistant, it is unlikely for a third party to have enough of a presence for it to gain any ground in this sort of election. That is, while two parties may trade back and forth for seats, a third party, especially a niche party, which third parties usually are, is unlikely to be able to carry any seat. It is this fact of voting mathematics that created the coalition systems you see in many other countries. It's what we effectively have in the US, too, it's just that the sub-party factions don't usually have names, nor do they usually change allies. Other countries have systems that correlate numbers of votes to seats in a congress, so that a party that gets 5% of the vote gets 5% of the seats, as opposed to the 0% they'd be incredibly likely to get in the US system.

It's this reason that Nader was accused of costing Gore the election in 2000. Few people who voted for him would have voted for Bush had he not been in the election, while many of them would have voted for Gore. That's the notion, anyway.

It would require a different voting system to get rid of the two-party system. I'm sure Dan would be happy to elucidate.

Actually, doing some research to make sure I had my terms straight (I didn't), I find that there's a law that states exactly what I'm saying: Duverger's Law.