UPDATE 200802120955 PST: On my system (Ubuntu gutsy, wine-0.9.47) this seems to be working. I will leave this page up for posterity. --drink
Since about 0.9.13 or so Wine has been capable of running Steam and its attendant games, which includes items like Half-Life 2. However, steam backups don't seem to want to work properly. When you run them, you get the following dialog:
This is quite annoying when you're trying to install a game. Especially if you don't want to download 5GB over a modem.
I tried some various things, but I found the most important clue in wine bug #3923, "Steam backups only work from steam directory". That is not all you need to know; at least, it was not sufficient in my case, with Steam 39 / 378
In my case, the solution was to change to the directory where the steam backup lives — .../name_of_backup/Disk_1 in each case for me — and create there a directory called c:. Then inside that there is a directory program files (note lack of capitalization) and in there a link from the Steam directory ($HOME/.wine/drive_c/Program Files/Steam) to steam (again, lower case.) First making sure that Steam.exe was not running, I then ran wine steambackup.exe (double-clicking the icon did not work) and restored the backup as normal... normal, except that Steam.exe (which would now successfully autolaunch) would hang at the end of the install. However, after killing the Steam.exe process, and restarting steam, the games would be installed and updates would proceed.
I'm now having problems with Steam.exe hanging, but I did successfully manage to start Half-Life 2. I didn't play but I did wander through the menus and set my graphics options. This included support for my panel's native resolution (1680x1050) and it looked smooth. The intro animation (Valve) had incorrect color (looked like 255,0,255 coming through the video in alpha'd regions around the edge?) but the game graphics appeared correct. Also, after Steam is quit or killed, it seems that in order to re-run it it must be run from the Steam directory with wine Steam.exe. ./Steam.exe did not work, nor did the icon.
See the Steam on Linux Howto's section on logging in (and the subsequent section on command line options) for information on how best to start Steam and Steam games.