I have come across this post in GamersReports that says that according to a post on XBox.com, there is an internal Best Buy document that says that new Xbox360 Premium systems will include a 100GB HDD instead of the previous 20GB HDD!
I dunno whether is this rumour is going to turn true, but if it does, it is indeed going to be a huge step up. I agree with GamerReports that the only reason we can see MS doing this is if that Direct TV downloadable TV show rumour was also true.
However, don't believe this yet, the status of this news is rumour still.