The thinking is that demographics predict Obama's performance (blacks and college-educated whites help him), and Obama's performance serves as a rough proxy for how Democrats will do this time around. It's a neat way to identify races that may become unexpectedly competitive with Obama at the top of the ticket.
So if you're represented by a Republican in the House, take a look at the spreadsheets (embedded into the page by the magic of Google Docs) and take a look at how the demographics project Obama's performance, and how winnable your district is. We've won three straight special elections in places where Democrats don't usually win, so it's a good year to go after the local GOP congressman.
Also, a word to any Democrats suffering under Mike Rogers, the Republican in Michigan's 8th district -- your district comes in as the 18th most competitive! Please try to make sure your challenger's website doesn't look like this. And that number's been stuck at 2880 each time I've checked it tonight.