Right now he shows 83 House Dems in competitive races and only 23 Republicans. He uses the standard categorization: Likely D, Leans D, Toss-Up, Leans R and Likely R. That yields this very nice House Summary Chart that right now shows a likely Republican pickup of 27 seats with the Dems holding 19 out of 20 of the Toss-Up seats. Get half of those and it's a pickup of 37 seats.
...
See this:
http://www.centerforpolitics.org/crystalball/articles/aia2010022501/
This guy (a statistician and contributor to Sabato's outfit) predicts the same outcome (-37).
His best case for Dems is -20 and their worst case is -54.