Gizmondo has an excellent review of the DS-Xtreme media enhancer:
I got my hands on the DS-Xtreme media enhancer for the Nintendo DS and DS Lite that we first told you about back in August. This "enhancer" also has the ability to play game backups and other media files on the DS. This card has 512MB of internal memory that can store homebrew application, game backups and audio files. This is the first DS mod that is truly painless—it is entirely plug and play compatible. The cartridge has a miniature USB port that plugs into a PC, the PC recognizes it as a flash drive and simply drag and drop the media onto the DS-Xtreme.
Except for the price this sounds like a great idea. Hopefully such a device will encourage home brew developers to create some great games rather than lead to rampant piracy.