[Updated below with confirmation from Valve.}

It's one of the worst-kept secrets in the video-game industry, it's getting lots of Mac gamers very excited, and it just got even more interesting.

Bellevue-based Valve Software has been dropping some not-so-subtle hints about plans to bring its popular Steam video-game store, distribution system and software client to Mac OS X -- teasing Mac gamers with the possibility that they'll soon have native access to such blockbusters as Half Life, Left 4 Dead and many others.

Over the weekend, a leak from an upcoming issue of Gameinformer magazine, reported by The Unofficial Apple Weblog, reiterated the news and went a step further -- suggesting that people who have already purchased games on the Windows version of Steam won't need to pay again to play the same games on their Macs. TUAW couches that part by saying it hasn't been able to confirm it, but the rumor is spreading quickly among game-starved Mac users, so Valve may have a PR situation on its hands if it's not true.

In whatever form it's rolled out for the Apple operating system, the Steam platform could help give additional momentum to Macs, boosting their profile in the video-game community. In that way, this will be an important one to watch for its potential to affect the broader balance of power between the Microsoft and Apple ecosystems.

Update Valve confirmed all of the speculation in a news release today.

"Our Steam partners, who are delivering over a thousand games to 25 million Steam clients, are very excited about adding support for the Mac," said Jason Holtman, Valve director of business development, in the news release. "Steamworks for the Mac supports all of the Steamworks APIs, and we have added a new feature, called Steam Play, which allows customers who purchase the product for the Mac or Windows to play on the other platform free of charge. For example, Steam Play, in combination with the Steam Cloud, allows a gamer playing on their work PC to go home and pick up playing the same game at the same point on their home Mac. We expect most developers and publishers to take advantage of Steam Play."