Parker Griffith, a Democrat from Alabama, is going to become a Republican. The practical implications of this seem to be basically nil—Griffith voted against health reform, voted against the clean energy bill, voted against financial regulatory reform, voted against the stimulus, and at least talked about voting against making Nancy Pelosi speaker.
But the implications are still bad. People rarely volunteer to join the minority.
Mostly, though, I think this is a reminder that the Democrats’ current huge majority with 257 members isn’t remotely sustainable. To get a majority that big you need to win a lot of districts you just can’t reliable win. Substantial losses in 2010 and/or 2012 are basically inevitable. That said, there are still a few GOP-held House seats that could plausibly be won by a reliably liberal Democrat. The real issue is whether the Democratic majority can add a few seats like that, and contain losses enough to maintain 220-230 reasonably reliable votes and thus the effective ability to govern.