This one sits firmly – VERY firmly – in the rumour camp, but Kotaku has picked up some conversations in the Steam Forums where users had found some support in the new beta release for Mac-specific files.
Kotaku puts most of them down to the switch over to a Webkit-based interface, but there are still a few file types that aren’t accounted for.
Even if the service does launch for OSX, it won’t have the same library as PCs, so Mac users will probably still have to boot Windows on their Macs anyway.
In any case, if anything does get announced, we’ll let you know.
[Kotaku]