I still find it amusing that the political spectrum is viewed as a line between two diametric opposites, when the two currently-reigning parties are almost indistinguishable these days.
I have to disagree on, well every word you said really.
The US political party system, as it stands, works like this: there are two major groups, the Republican Party, and everyone else. So far as I know, no independent has ever caucused with the Republicans.
Were we in Britain, the Democrats would be "leading a coalition government" in the senate next year. The reason we do not use that term is that there hasn't been a viable national third party since the Populists, who you'll note currently don't exist and have almost everything they wanted.
Second, the political views of people are generally viewed as having 5 possible differing opinions (radical, liberal, moderate, conservative*, and reactionary) on a myriad of topics (international, social, etc). Is this accurate? Not really, but it serves well as a way to predict voting since it closely mirrors how you break up a bell curve.
As for your second statement, that the parties are indistinguishable, I present the 2004 party platforms without further comment.
Socialist 2006: http://sp-usa.org/platform/
Libertarian: no offical national web source found
*Spell check suggests: constrictive.