It is a sure bet that Paul Thomas Anderson's excellent film There Will Be Blood will grab a few Oscars tonight, but unfortunately it won't get one for Best Original Score. The reason is an arcane and stupid Academy Awards rule excluding "scores diluted by the use of tracked themes or other pre-existing music".
The score was written by Radiohead guitarist Johnny Greenwood, combining pieces by Arvo Pärt, Johannes Brahms, and others, with a good chunk (supposedly about 35 minutes) of music written by himself specifically for this film. He also used a few pieces of his BBC-commissioned 2006 composition Popcorn Superhet Receiver (RealAudio streaming link). Apparently this was too much "diluting" for the Academy, and it really is a shame that this keeps the best 2007 film score out of the awards.
If you think this is a 21st century problem, you might be surprised that this rule has already robbed another great composer from receiving an Oscar, 35 years ago. Nino Rota's score for the original 1972 Godfather movie was ruled ineligible for recycling a theme from his soundtrack for the 1958 film Fortunella. However, Rota used the same theme again in 1974's Godfather II, and this film finally got him his deserved award. It doesn't make any sense, but it really happened that way. I am waiting for There Will Be Blood II...
You can listen to a track from Johnny Greenwood's score on his MySpace page, and you should also check out an excerpt from Bodysong (YouTube link), a 2003 film by Simon Pummell which was scored by Greenwood.