There's nothing quite like a friendly visit from one of the biggest companies on Earth to get the speculators' engines running. In this case we are talking about AppleInsider.com's unconfirmed report that Apple CEO Tim Cook paid a visit to Gabe Newell and the rest of his team at Valve's headquarters in Bellevue, Washington this week.
If the report is true (and the story offers no evidence to back up its claims), then there could be any number of reasons why Cook would want to visit Valve. It may just be that Cook is a big fan of Portal or the Half-Life franchise and wanted to meet the people behind those great games. On the other hand it's much more likely that Cook and Newell talked business during the visit. People are already speculating that Apple and Valve could be collaborating on Valve's still somewhat mysterious hardware project or that Cook wants to see more of Valve's games on the Mac.
The article does point out that Newell has been highly critical of Apple's attitude to game development in the past. However, Valve later released Mac ports of many of their games in 2010, along with support for Mac game downloads via Steam that same year.