Kevin Drum shows a chart that suggests voter party allegiance is decided when they turn 20, based on whether the current president is popular or not. So, people who turned 20 when Reagan was president are likely to be Republican, whereas people who turner 20 when Clinton or Bush Jr. were presidents are likely to be Democrats.
On some level I guess this is good news, in that it proves the existence of SOME sort of a feedback loop (i.e. bad politicians/parties will be punished). It's just a shame that it may take generations for the feedback to show up in election results (as the 2004 elections showed - it didn't matter how much Bush Jr. screwed up, we still have to wait for those Reagan-era youngsters to be replaced because they're not going to change their minds).