This is hugely significant - as well as the groundswell of great indie games, many of which are on Steam already, Steam brings some top-tier games such as Portal, Half Life and potentially many others. Valve, the owners of Steam, have always been quite Linux friendly (using it for dedicated game servers run by individuals) and now have confirmed they will ship Linux games.
The great thing about Steam in this context is that, as with Windows and the recently added Mac, you can initially run games on one platform and (assuming they are supported) instantly run them on another, e.g. use Portal on PC today and on Linux when it's available, without paying extra. In fact all Steam games can run on any mix of Windows PCs, Macs and (soon) Linux.
Valve also sell the Source engine for 3D games, which while a little behind the cutting edge is commercially viable, and has the benefit of not requiring very high end hardware. This major game engine will also be supported on Linux, making it much easier for third party game developers who already use Steam and the Source engine on Windows to support Linux.
Steam does use DRM but it's entirely network based - no hidden sectors on CDs, no fingerprinting of PC hardware. You just supply the Steam username and password on any PC and you are able to install and run the games in your account. Of all the various DRM systems it's the least objectionable. The Crossover Games and WINE community has been supporting Steam games for quite a while now, because it's quite easy to have Steam work on Linux.