You know, it hasn't been announced or anything, but you know what would make the Steamworks announcement make even more sense?
If they were working on a Test Chamber Creator for Portal 2 as well. A simple, straightforward version of Hammer that takes care of all the bullshit in the back end, and makes it easy for normal users to create your own twisted hellhole puzzles to challenge your friends with... and get shared out to them via Steam.
Sort of like LBP: Portal Edition.
The Steam integration would let them push user-created content to consoles without having to go through any sort of certification process.