OK I said it.
But seriously though, a smart move by Valve if they can pull it off. There is an untapped market for real games on Linux. They would be the only game (pardon pun) in town, and many/any linux user would likely be more than happy to fork over money for games that run naively on Linux, particularly given the cost structure and the slick nature of remote distribution by Steam. I would even go so far to say that many, having saved 100-400$ by not having to buy a Windows OS, would justify or rationalize spending more on Valve games, and this is "Free" money (i.e. they saved it initially, so why not spend it on things that are fun, like games).
So while "Year of the Linux Desktop" it might not be, it is certainly a step in the right direction, as well as a step towards OS parity.
In another note, it might be interesting to see if this effects netbooks. Steam has a lot of independents, and smallish, low horsepower type games that are really cheap. Netbooks can only install stuff via networks (generally speaking). So think of a netbook, running Linux, and Steam, and using that service to install netbook friendly games onto netbooks. An interesting combination I think. Particularly given the low cost nature of all the above... low cost netbook, low cost linux, low cost gaming... Definatly a niche, but perhaps a very popular/profitable one. Look at iPhones for example.