Worked out fine for me, downloaded and installed Portal, already played it a bit, 'till level 12. Also bought Torchlight, but have yet to play it (but looks promising, I miss D2, and love Dungeon Hunter on my iPod, so hopefully this has a good balance between those).
The whole point of launching in stages is:
1. Display a certain Steam technology each week (this week is Steam Play, so buy on Windows, play on Windows or Mac, and later Linux)
2. Not to overcrowd the user with choice. Most people will be happy with one or two new games, and a lot of Mac users are already casual gamers, so TF2 and the like are too much for them. (I think)
3. Give them time to test the system on a massive scale. (I think as well)
Oh, and Steam for Linux should be out soon, they've shown intent, and Phoronix has reported on it. I think they've even run it on Linux, to compare graphical performance under OpenGL (Windows > OS X > Linux at the moment, though Linux hasn't got a full client, OS X hasn't got a full implementation of OpenGL yet, and Windows version is far more stable).