It will be available together with the Nintendo Wii on December 8th.
The SD card-based player is fairly well self-contained, mainly leeching off of the DS for controls and power -- it even has its own headphone port -- but that means it can also work with the less-powerful Game Boy Advance/Advance SP/Micro lineup, which is always a plus. Sound can at least be routed through the DS' headphones and speakers.