Prepaid codes can be used to extend an Xbox LIVE subscription or redeemed for games, avatar items, and special offers.
Notes about prepaid codes:
Xbox 360 console subscription codes are designed specifically for the Xbox 360 console and are not compatible with the original Xbox console.
Prepaid codes cannot be consumed or activated if the user has billing violations or their payment instrument is in decline. The user must first remove any violations before utilizing a prepaid code"
So if it's any indication, the 720 will probably have its own codes and own system. Pure conjecture on my part though.
It's sound conjecture. These cards may as well be $35 MSRP as they've been on sale like this for a couple months now. These cards will either not work with the next Xbox, or the next Xbox's online service will allow free online play and charge for a different set of features.
It'll be interesting to see if these Live cards go on fire sale after the new Xbox is formally announced.