It’s a bit of a jump to assume so, but Sony and Valve’s exclusive partnership would definitely not be in Microsoft’s best interest. Microsoft has announced that it will integrate RakNet’s technology into the existing Xbox Live interface. You may ask “why should I care at all?” You should care because RakNet’s tech is what made Steam compatibility possible on the PS3.
Microsoft’s inclusion of Raknet’s technology allows for Xbox users to get more out of their console than ever before. The technology allows for lobby and room matchmaking, Network address traversal connectivity, and the most important feature is the use of Steam servers for forwarding traffic.
Basically what it all boils down to is that this is possible. Is it likely? Hell no. There is no indication whatsoever that Xbox 360 users will get the Steam treatment, but maybe they’ll get some exclusive options of their own. If Steam does happen to make it’s way to Xbox 360, there’s no doubt there would be a strong market for it.