Microsoft announced today that Portal: Still Alive will bend reality on XBLA beginning next Wednesday, October 22nd. The title includes the original Aperture Science adventure (Joystiq's GOTY 2007), along with 14 new "exclusive" challenge maps and six advanced maps, complete with new Achievements. So, you can have your cake and eat it, too. (Damn.)
This isn't to say Portal: Still Alive is the only title coming next week to XBLA. The previous two weeks have had additional games announced on Wednesday.
Source - Announcement [Gamerscore News]
Source - Portal: Still Alive Achievements [Gamerscore Blog]