The Monkey Cage shares some data indicating that the number of independents in the US is vastly overstated by the usual-suspect pundits:
Most independents are closet partisans. This has been well-known in political science since at least 1992, with the publication of The Myth of the Independent Voter (here). When asked a follow-up question, the vast majority of independents state that they lean toward a political party. They are the “independent leaners.”
I think the current health care reform debate underscores that the "rise" of independents is really a rise in the number of liberals and conservatives who don't simply don't like the political process -- with its compromises, vote-trading in Congress, etc. -- and thus refuse to identify with the party that's closer to their views. This is bad for both parties, but worse for the Democrats, since they're more likely to try to pass substantive (if not perfect) legislation. The idea that both parties are corrupt and not to be trusted helps the Republicans, since they're more likely to try blocking legislation.