Basically, expect the feature set of the PS2 version, but with five added songs. No online, no downloadable content… at least not yet. Sounds like that could change in the future.
Here’s the official to my questions, from a “Rock Band” spokesperson:
“In response to your questions, ’Rock Band’ for the Wii will have functionality similar to the PS2 version. There will be no DLC available, but the Wii version will have the five additional songs on the game disc. This version was also co-developed by Pi Studios [and Harmonix].
“When developing ’Rock Band’ for the Wii platform, we focused on making the core gameplay experience as solid and enjoyable as possible while tailoring it specifically to the strengths of the platform.
“The Wii version still contains the robust four-player band experience and all the fun at the core of other versions of ’Rock Band.’ However, because the Wii’s online capabilities and potential have yet to be fully realized, we wanted to wait before we explored online functionality for ’Rock Band’ to ensure that players get the high-quality of online performance they’ve come to expect.”