[quote name='extremep']Has anyone seen any details on how they're going to handle this? Can existing or new Live accounts created on Xbox 1 be used immediately on Xbox 360 without any transfer needed? And, what is Live going to be like on Xbox 1 after Xbox 360 is out? Will Live Silver (free multiplayer on weekends) be available for Xbox 1 owners too?[/QUOTE]
Existing XBL accounts can be used immediately on 360. You'll probably have to enter your account information or something.
Live is going to continue for both Xbox 1 and Xbox 360. Its the same service. You can play Xbox 1 games on the 360 against people playing them on Live with the Xbox 1, as long as its backwards compatible.
I don't know if Silver will be available for Xbox 1. You do get a free month of Gold included with the 360, though.