Now, obviously there are serious legal issues to be considered here and I'm guessing EA will ship the title without a feature that would allow players to create their own songs knowing full well that doing so would immediately open the doors to Copyright infringement issues. Clearly this is something EA wants to avoid given how closely they are working with various rights holders.
Whether Rock Band supports it or not, though, the fans will eventually find a way. I've recently come across this tutorial for uploading your own custom songs to Guitar Hero II (PS2 only, I believe). You can see an example of this in action in the video embedded below.
Copyright issues here are obviously less of a concern given how complex it is to upload one's own songs. Only the most dedicated players would invest the time and money to do so.
Its a pity really -- I'd still like to see a way for players in Rock Band to be able to easily create their own content in order to get the true 'band' experience in a way the copyright holders could accept. What do you think? Is it even possible? Or would user generated content in Rock Band be a Pandora's box of legal issues and/or crippled functionality?