I'd heard long ago that the first 4 bars or something like that had to be unique. Which would be why a lot of songs start with 4 bars of G or some other chord. Probably just an old wives' tale, though.

I agree with JonChorba. I've always wondered, with 12 notes, how anything could be written today that hadn't already been written sometime in the past. There's probably 10/7,000,000,000 on any given day writing the identical riffs. :D