It seems that the partnership between Valve and Canonical is stronger than what we were led to believe, especially if we take a look at the About page from the official Steam website.
Right from the get-go, Valve announced that they were developing their Steam client to be compliant with Ubuntu 12.04. It wasn't a mystery why they've chosen this platform. It's the most common Linux operating system in use, today.
Canonical also has a lot of partnerships, with Dell and Lenovo for example, so it's not surprising that they've managed to get an exclusive partnership with Valve.
Steam for Linux can also be run on other platforms and Valve stated that they were going to improve the support for other operating systems.
Nonetheless, only Ubuntu 12.04 is recommended on Steam's official website and a link is provided to Ubuntu.com.
It's going to be interesting to see how Canonical will manage to captivate Steam’s huge user base.