Not just shooters, even with the existing amount of already Mac-ported games out there, this sends a message to developers that Valve is taking Mac development seriously as a Tier 1 platform and it has the tools to assist in making ports easier for their service which already makes games easy to buy and sell. The rumors which are swirling around the net since the official announcement may indicate several other games with Mac versions ready at launch.
I’ve been enjoying more Steam games since Boot Camp on Intel Macs, and not needing a separate PC. They really do offer the largest catalog, great deals, some demos and free trials of upcoming titles here and there, and they go ape-shit bananas slashing prices over holidays and giving out bundles. I’ve gifted some cheap mini games to friends. But as always, I usually wish I didn’t have to boot into Windows to play them. Besides web development testing, it’s all I’ve used Windows for otherwise when no Mac version of game X is available. It’s nice to see Valve is finally seeing what Blizzard saw long ago.
I know in recent years the PC as a gaming platform has dropped off, mostly in retail sales. Some publishers have been very nice pricing new PC titles at a reduced cost compared to a console version, or giving away DLC stuffs in order encourage buying over pirating (or also because the lack of licensing fees). However digital download and community services like Steam continue to prosper very well. I believe Valve saw the writing on the wall after Microsoft fucked up the Games for Windows branding and began to shift their focus more to the XBox. Yes, GFW Live is pretty wretched, almost nothing like the Xbox community which it was modeled to mix with. It’s obviously in Valve’s best interest to reach the largest possible market now. Perhaps also as a stepping stone into the iPad/iPhone market later down the road, who knows?
I own most of the Valve titles already for PC. Picked up the Orange Box for cheap when Circuit City was closing its retail chains, as well as the original Left 4 Dead. Will be awesome to get Mac versions of all these, and I may just go ahead and get Left 4 Dead 2 and Portal 2 (by then) when Steam makes its debut on the Mac. Maybe…. just maybe I won’t even need Windows for gaming soon, and then hell will have truly frozen over. (I’m not a console person either.) 
