Yesterday, the Valve Linux team publicly announced their ongoing work to bring Steam to Linux. A major part of that announcement is the choice of Ubuntu 12.04.
Valve has been a major force in gaming since 1996. Gabe Newell and the Valve team have created some of the best game series EVER. Half-Life, Counter-Strike, Day of Defeat, and most recently Portal are extremely popular, and quite addicting.
The one thing missing early on was a good distribution mechanism. Valve learned early on that retail physical box distribution can only go so far and was expensive. The Steam client came out of hiding in 2003 and has been a driving force ever since. Many game platforms have tried to create what Steam provides from multiplayer communication and community features but none are as strong.
The linux gaming community has been very vocal in trying to get more support in the gaming community. With the growth in numbers of independent developers, the number of indie games supporting Linux growing exponentially, and quite popular game engines such as Unity3D supporting Ubuntu, Valve finally came to the conclusion that even their game engine Source has to come to Ubuntu.
The announcement states there is a 11 person team working on bringing the Steam client to Ubuntu and the first game will be Left 4 Dead 2. This is yet another huge development in the gaming space for Ubuntu users.