Steam has had a serious problem for quite some time, failing with a strange debug assertion error popup at the dreaded 27% mark.
It would be great if steam worked with wine. I did some testing some time ago, and Steam seemed to work except of this problem. I used cvscedega to work around the 27% Problem and after that I ran it with wine.
It worked quite fine, registration, game registration, game setup and game start. I had MSIE installed and had to use a few native dlls(cryptography dlls and IE friends) but it seemed to work fine with wine.
The next problem was not wine releated and is somewhat harder to fix. Steam turned out to be really download hungry. I just installed it and updated Half life and Steam had pushed 100 MB through my internet line. I've a download limit of 600 MB wich is really a small limit. One day I trashed my wine installation and didn't re-install steam afterwards.
If I find some way to download these things at school I can help you with fixing steam!
Stefan Dösinger