According to this, Harmonix's Greg LoPiccolo has given a clear indication that Rock Band will allow users to create their own music at some point in the future.
"We would like to make it possible for people to introduce their own music into sort of the Rock Band ecosystem, which is a pretty complicated topic," he said.
"It isn't easy to achieve - there are IP issues, ratings issues, and so forth. So there are a lot of things to work through to get that to work, but that seems like a natural direction for the franchise to move. We'd love to have it be a vehicle for people to be able to express themselves in terms of creating their own music and bringing it to a wider audience."
There's still no European date announced for Rock Band, but the truth is out there now. Doesn't sound like you'll have long to wait.