From daily observation of SleepBot's rankings...I found a few interesting things about the way Google ranks the top applications on the Android Market.

Google's potential heuristic function for ranking:

Definition of Recent: past week/month.

Features/Weight. 1. Recent Downloads / 20 2. Recent Keep Percentage. / 30 3. Total Downloads. / 20 4. Total Keep Percentage. / 30

AppBrain's top rated is rather simple: 1. Ratings. 2. Total Downloads.

(To be edited later)