Well, this article from Giantbomb says that they do want to try and release some of the more popular RBN songs to the Wii as well as the PS3.

It seems like the whole Xbox Creators Club is key to the process at the moment. Without any obvious analogs on the PS3, PSP, or Wii, how will you bring Rock Band Network to those platforms?

LoPiccolo: It's our intention to provide selected material from the Rock Band Network on both the PS3 and the Wii, and we're engaged with both Sony and Nintendo to figure out specifically how to bring that about. We would like to make at least the best material that emerges on the Rock Band Network available across all platforms.