Portal: Still Alive for the Xbox Live Arcade in an interview with OXM. "It's just a series of new challenges," Lombardi told the magazine. "[Portal: Still Alive is] another way for people to experience Portal, without having to buy the full Orange Box."
We can't say we're surprised. With such a focus on story we didn't expect Valve to do anything exclusively for the Xbox that would factor into actual canon of the series.
Lombardi later detailed that Valve has new projects in the works for the Xbox 360 but remain firm on its disinterest for PS3 and Wii development. It was also confirmed that Team Fortess 2 that the achievements and content added to the PC version of the game will come to the Xbox 360 at some point, when Valve has worked out how it wants to package and release that content.