I could see this being problems for Steam, unless MS and Valve partnered up (which I hope won't happen). Developers look and see, hey, now we don't have to make both a PC version for Steam and 360 Version for Consoles, lets just make a 360 Version!
Although, lets look at something else here, Valve may already be moving to counter them. Small amounts of Steam compatibility came with the PS3 Version of Portal 2 for the PS3, and I remember reading or hearing somewhere that PSN would be replaced by Steam in the next gen Sony console (don't know how true that is, but I think I remember reading it somewhere reputable). So, what if then every PlayStation game could also be played on Steam because of this event to keep both Sony and Valve competitive with Microsoft? Then PC Players are the real winners, we get Steam, we get PlayStation exclusives, and we get XBox exclusives. Win :biggrin:
I don't know if thats how it would pan out, but I wouldn't be hugely surprised if it did do that, since Sony and Steam would have to do something to keep a competitive edge.