I know many people on diaspora using windows only for steam
You are probably correct.
The big problem is not porting Steam but porting the games people want to play!
There are oodles of games for Windows on Steam and they have done a pretty good job with most getting them to be compatible with Windows XP through to 7 x64, but the choice for Mac is much smaller and I can't imagine that any time soon a huge library of Linux games will appear.
There is also the real problem distro differences to deal with - unless they abstract that - and then it would be good if graphics and sound card manufacturers actually started to produce any drivers for Linux at all.
Not an easy task for Steam to convince the developers who have to see dollar signs (or pound signs, or whatever) before committing themselves to a very expensive project.