Check your e-mails people. I just got my music beta invite like 20min ago. I don't have a XOOM nor did I attend I/O. Couldn't believe there wasn't a thread up yet.
EDIT: Thought I'd share some details
-Really fast streaming with a nice clean PC interface
-Music Manager program seems to only be for uploading, the Music Beta player itself is browser based
-20,000 song limit; don't think I'll be filling that anytime soon
-Nearly 150 free tracks already there for you