Can you really play with backups over Xbox Live? You couldn't even do that with the original Xbox if I remember correctly... could be wrong though since I never wanted to try it for myself and only read about it on the Xbox-Scene forums numerous times.
If you can play with backups over XBL, then I'll be grabbing a 360 as soon as the drop the price, which is hopefully soon.
EDIT: After quickly searching on google for playing 360 backups on XBL, it seems you are right and as of right now you can. I know that Microsoft just released the big Spring 2007 XBL update, so I wonder if that has had any effect on this at all.