Breakthrough! So I've been working on some movie-based correlation ways of predicting ratings (if movie A is "like" movie B, and user U likes movie A then she'll probably like movie B). After some tweaking, I got some probe data to have an RMSE of 0.981815, which is just barely off the leaderboard (although the real data will be slightly different, so maybe I'll make it on!) I just finished computing the correlation scores for the real data (took about 24 hours of computer time) and tonight (after WoW) I'll start doing the ratings for them, so I might be able to submit as early as tomorrow.
I have some ideas of what to try next, but it's nice to see that I'm definitely making progress. It would be awesome if I were on the leaderboard again :-)