TopMuffin doesn't just post availability info, they also do some statistics to find the percent available. When we checked the page, Apple had 45% availability for 8GB black iPhones, 53% availability for 16GB black iPhones, and 41% availability for 16GB white iPhones.
This site does not list availability for AT&T stores, and only lists stores located in the U.S. A disclaimer is posted on the page lets viewers know that the best way to check iPhone availability is to call your local store. You can view the site by clicking the read link.
[via Daring Fireball]