I just installed play on linux (4.0.14, on kubuntu oneiric).
Under play on linux, I installed steam, connected to my account, downloaded and started left 4 dead (1st, not 2nd).
I have following problems:
1. when l4d is configured to run on full screen, it tries to run on both screens, which ends up that it shows on 1 screen only, but everything is compressed - so the actual action is on half of screen only.
2. i switched to windowed mode, 1280x720, and it works ok - i.e. i see everything, but i'd prefer to have it full screen - on one monitor.
3. l4d is slow. very slow. i mean - when I move, it generally moves, but then it pauses for a while, and then starts moving again - no way to play.