I have been messing about with this for hours and sort of have Team Fortress 2 running. Everything locks up if I try to change resolution in-game, or go between windowed and fullscreen mode - I have to power off the iMac and reboot. If I use command line options for fullscreen in any resolution other than the monitor's native res then the game appears half off the screen.
So by specifying:
-fullscreen -w 1920 -h 1200
..I get something approximating the experience under Boot Camp. Except that the frame rate is about half that of running Windows natively, and a large part of the ground is invisible! (And there's the issue with the chat text, as you know.)
If I run TF2 in a window at a lower res, with all the options dialed down, then the performance is acceptable, except that it's tiny and the ground is still invisible. (Steam also complains that the video drivers are out of date when I launch the game.)
Hope this helps!