http://bugs.winehq.org/show_bug.cgi?id=26835 just got a fix committed to wine git, it was responsible for Portal 2 not launching in Wine without a cracked binary that sidesteps some of the DRM.

I believe there's still some instability issues, but the game should at least be playable now

I haven't tried it myself, I'm lazy enough to wait a week or so for a dev release to appear in the ubuntu PPA, but I haven't played the game on Windows since my first single-player playthrough due to the same laziness...