This just in from Google I/O: A system dump of Jelly Bean from a GSM Galaxy Nexus. You can run over to RootzWiki right now and grab yourself a copy.
Somewhere, deep in this zip file, are all the goodies we saw today at I/O. Expect bits from this to be chopped, ported, and crammed into existing devices shortly.
I'm going to get cracking on an emulator setup for a Jelly Bean edition of Getting To Know Android.