It'll hit the 360 in the form of downloadable content, and includes everything you'd find in the PC/Mac version of Peggle Nights. Which means 60 new MP levels, 60 new campaign levels and 60 new challenges, along with more achievements.
It'll be out sometime this Fall. It's Peggle. More Peggle. You know what to do.
Peggle XBLA Getting 'Nights' Expansion [Shacknews]