picture of the Rock Band Wii drum kit before, IGN has gotten its hands on the final retail product. Beyond the obvious Wii white color difference of the drum set, the site says that the drums come with the "upgraded" components currently packaged with the PS3 and Xbox 360 versions.
The upgrades include a strengthened kick pedal and "somewhat" softer drum pads. For people who bought one of the original copies of Rock Band, they would notice a thicker spring beneath the Wii drum's kick pedal. The site concludes there isn't much difference in the hardware, but the Wii version has no DLC song option. If you have the option of purchasing Rock Band for another console, it's probably best to do so.