Additionally, Valve has now updated the list of games that "offer cross-platform support, regardless of purchase date" to include Fairway Solitaire. Assuming the list is now accurate, that means all of the Steam Mac games Valve's promoted thus far are available to previous Steam PC owners without the need for an additional purchase.

[Original] - When Valve launched Steam for Mac yesterday, the company boasted a list of over fifty launch games for the digital distribution platform, noting that one could play any of the launch titles "on all Steam supported platforms" with a single purchase.

However, it turns out that this functionality, referred to as Steam Play, doesn't always extend to all of one's past Steam PC purchases--if you bought a game on Steam PC before it was eligible, you may have to buy again if you want it on Steam Mac.

Thus far, only two of the fifty-plus offerings--Civilization IV: The Complete Edition and Fairway Solitaire--require existing Steam PC owners to rebuy them on Steam Mac if they were bought before "being eligible for Steam Play support (May 12th, 2010)."

To help make matters more clear, Valve issued the following list of the current Steam for Mac games that "offer cross-platform support, regardless of purchase date."

• And Yet It Moves
• Atlantis Sky Patrol
• Bejeweled 2 Deluxe
• Bob Came in Pieces
• Bookworm Deluxe
• Braid
• Brainpipe
• Chocolatier: Decadence by Design
• Chuzzle Deluxe
• City of Heroes: Architect Edition
• Cooking Dash
• Diaper Dash
• The Dig
• Diner Dash: Hometown Hero
• DinerTown Detective Agency
• DinerTown Tycoon
• Dream Chronicles: The Chosen Child
• Escape Rosecliff Island
• Fitness Dash
• Football Manager 10
• Galcon Fusion
• Gemini Lost
• Guns of Icarus
• Hotel Dash Suite Success
• Indiana Jones and the Fate of Atlantis
• Indiana Jones and the Last Crusade
• KrissX
• Loom
• Luxor
• Luxor 3
• Luxor: Mah jong
• Machinarium
• Mahjong Roadshow
• Max and the Magic Marker
• My Tribe
• Nightshift Legacy: The Jaguar's Eye
• The Nightshift Code
• Parking Dash
• Peggle Deluxe
• Peggle Nights
• Portal
• Professor Fizzwizzle and the Molten Mystery
• Quantz
• Sam & Max: The Devil's Playhouse
• Tales of Monkey Island Season 1
• Toki Tori
• Torchlight
• Trijinx: A Kristine Kross Mystery
• Unwell Mel
• Valerie Porter and the Scarlett Scandal
• Wandering Willows
• Wedding Dash 2: Rings Around the World
• World of Goo
• Zenerchi
• Zuma Deluxe

While all of the games currently available through Steam for Mac support Steam Play, Valve has previously cautioned that it may not be universally supported across the entire Steam catalog, noting the functionality would be available "for our games and any of our partners who are interested in using the feature." Thanks CoRDS