This just popped up at digg. Someone had the glorious idea to find the UPC barcodes for the Sony PS3 and the Nintendo Wii. You can print out the code and try to have a Wal-mart employee scan it for you at your local store. The Wal-mart inventory system should then tell how many Sony PS3 consoles (story on digg) or Nintendo Wii (story on digg) consoles the store gets.

There are some comments in the discussion thread that confirm that this works, but it is actually against store policy to reveal that information to a customer. You need possibly do some social hacking.
This is a great way to stake out the Wal-Mart stores in your location and pick the one with the most inventory. This will be the best store for you to camp out in front days before launch of the Wii or PS3.