http://bugs.winehq.org/show_bug.cgi?id=26835
--- Comment #39 from Eevee eevee.wine@veekun.com 2011-04-19 14:35:25 CDT --- (In reply to comment #38)
It seems that Portal 2 queries some Steam server (or maybe your instance of Steam) to do verification of your game instance, and a failure in said verification is causing the crash. I suspect GameOverlayRenderer is somewhat related, and there have been issues with client-steam server communication in the past (check other games that force authentication with Steam).
The first thing I did was try Team Fortress 2 and original Portal. Both work fine. I've actually had the overlay working fine in TF2, as well; never had to disable the dll until Portal 2.
So what are you doing differently from everyone else that makes this communication work?