On CAG, you can add your Xbox Gamertags, PSN usernames, and Wii Friend codes to your CAG profile. I think we should be able to add our Steam profiles too, especially since Steam has a huge trading community.
You know, come to think of it, you might as well replace the Wii Friend Code setting on the profile to Steam, since I've seen maybe 2 people on CAG actually post their Wii friend code.
I think it's a highly wanted feature, but it has low visibility. Maybe we can link to this thread to the main Steam thread on the next update (it's pretty cloose to closing).