Yes and no. Valve is porting some of its existing PC/Xbox game titles to run natively on the Mac platform, and most (or all?) of their new titles will be released simultaneously on PC, Mac, and XBox 360.
Further, Valve is bring it's "App Store for Games" to the Mac, which will encourage other game publishers to port to the Mac, and give them a solid sales-platform to do so on.
Carrying that train of thought further, they should integrate with Apple's App Store. Everyone on a mac uses iTunes anyway, why should they have to install Steam separately?
Because Valve don't have to give Apple a cut when people buy through Steam.
Steam is more than just an app store, though. It's a social network, provides automatic updates for games, etc.
For the record, when I use OS X, I don't use iTunes.