To get a better sense of this I've generated out-of-sample (OOS) election predictions for 2006 and 2008 based on district-level poll results. So, for instance, I used OLS estimates from the relationship between the 2006 polling averages (over the last forty-five days of the campaign) and the actual 2006 House outcomes to predict the 2008 outcomes, based on the 2008 polling values. For 2006 I used estimates from the relationship between polls and votes in 2008 to do a backwards prediction (a "postdiction," I suppose) of the 2006 outcomes. The idea here is to see how well outcomes in one year can be predicted using parameter estimates from another year.
Here's how closely the OOS predicted values followed the actual House election outcomes for 2006 and 2008, pooled: