A US DSi will play UK DS/DS Lite game carts.

A US DSi will not be able to download and play UK DSiWare games (these are downloaded through the DSiWare application which will be available to download to your DSi soon).

A US DSi will not be able to play DSi enhanced games (there are none yet).

Here is a quick link: http://www.eurogamer.net/forum_threa...read_id=146956