Just used http://twilk.com to put my friends' faces on my Twitter background. Check it out!
The name Twilk is a combination of Twitter and milk. Milk has nothing to do with the service other than the creator of Twilk really likes milk. It helps that the domain name is short, and was available.
How are people arranged on the background?
People are sorted by how much you mention their user name ( @ replies, retweets, etc ) in your Twitter feed. It goes from top to bottom starting in the first two columns on the left. If you mention people the same amount (or not at all), the order of those people will be random.