A favorite reference for election junkies like those of us who inhabit this site is, of course, presidential results by congressional district. Unfortunately, most states don't publish this data, but it's reliably churned out by a firm called Polidata every four years. It's a difficult task, though, because it usually involves crunching data on a precinct level (and also figuring out what the hell to do with absentee ballots), so Polidata typically releases its findings some time in March after a presidential races.
But the good news is that, working together, we can come up with some preliminary data for at least some states. There are at least three types of states where we can get this data relatively easily:
1) States with just one at-large district (duh), like Montana.
2) States which actually publish presidential results by CD, such as Virginia.
3) States where district lines follow county lines or, in New England, township lines (or at least follow them closely), like Iowa.
Where we need help (at this stage) is in figuring out which states fall into the second and third categories. I know California also releases results by CD, but I believe a few other states do as well. And Arkansas and West Virginia follow county lines, but some other states probably do, too.
Also, let's use this thread for brainstorming about other ways we might try to figure out presidential results by CD (short of acquiring detailed precinct-by-precinct data). Please share your thoughts in comments.
UPDATE: Jeffmd observes that some states offer easy-to-use precinct data, so where available, that might be an option as well. If you're aware of any states which provide this information, please let us know that as well.
UPDATE 2: I've created a public Google Docs spreadsheet that we can use to compile a list of data sources. Please feel free to input any helpful information you're aware of. Note that we're not looking for the actual numbers just yet - rather, we want to know where we can find the numbers (and what format - ie, CD, county, precinct, etc. - we should be looking for). And if you are adding a URL, please use TinyURL.