In some of the biggest gaming news to break on the Mac platform in a very long time, Valve Software has announced that its Steam gaming service will be released on the Mac on May 12, 2010.

The company announced plans to bring Steam and its game engine, called Source, to the Mac back in March – at the time, they said it would be released in April, so the release schedule has slipped by a couple of weeks. Steam enables Valve and other companies to sell copies through downloads rather than requiring customers to buy games in the store.

A cornerstone of Steam’s release on the Mac platform is a new feature called “Steam Play,” which lets users buy a copy of a game they can play either on the Mac or on a Windows PC free of any additional charge. As envisioned by Valve, a gamer can play their Steam games on their PC in the office, then go home and play the same game at the same point on their home Mac.

Valve declined to identify which games they’ll be launching on May 12th, but they previously indicated that games including Left 4 Dead 2, Team Fortress 2, Counter-Strike, Portal and the Half-Life series would be available for Mac users.

Valve is doing native versions of its games – the company is not relying on what it calls “emulation,” or translation layer technology, the system used by some companies like Electronic Arts to bring out games for the Mac like The Sims 3. Valve’s director of Steam development, John Cook, called the Mac “a tier-1 platform” and said that Valve’s adoption of WebKit, the same technology that powers Apple Safari, and OpenGL, the graphics technology used on the Mac, as key to that effort.

Valve also indicated that Portal 2 – a first-person action game with puzzle elements, the sequel to a title that was released in 2007 – will be its first simultaneous release for Mac and Windows.