As it turns out, the kiosks aren't just for trading in games for store credit -- you can buy, trade, or even rent used games or movies from the device. The one we looked at had a tiny selection of both games and movies available (less than fifty total offerings), but that's most likely because of the early stage of the program.
We snapped some pictures of the kiosk while we were trying it out. Head into our gallery if you ever wanted to see what a game case deposit slot or a disc dispenser looks like.
The game trade-in component accepts only PS2, PS3, Wii, Xbox, and Xbox 360 titles. In other words, nothing too old, and nothing that doesn't use a normal-size disc (Gamecube, DS, PSP). The DS game we scanned to be extra sure went unrecognized. We went through (most of) the process of trading in a game, stopping only because the kiosk offered just 95 cents of store credit for our copy of Stretch Panic. At least we know the database is complete enough to know semi-obscure PS2 launch titles.
In any case, we can report that the trade-in process is quite simple. You just scan the barcodes of the items you want to trade in, then "check out" on the touch screen when you're ready to finish. The machine instructs you to put the discs into a disc slot (which apparently checks the condition of the discs) under the screen(s), and the cases in a different slot on the right side of the machine. Though cases are required for your games, don't expect a case with a game you purchase from the kiosk. We were informed that purchased games come out in paper sleeves.
Best Buy's CMO said that not all of the machines will have the capability for users to buy or rent products, but the trade-in portion seems like the real focus anyway, and the instant-gratification aspect of receiving store credit immediately for your used games, without having to deal with even a single person, may draw people in.