Valve has recently released Portal 2 (beta) for linux, I installed it, opted in "beta" under properties (this step is required for the game to launch) and gave it a try.
My machine is a Toshiba satellite L850-1l1 running arch linux x86_64, mounting a Radeon HD 7670m card.
Using the closed source catalyst (fglrx) driver (both 14.1 and 14.2beta) the game launched and displayed the menu correctly, but crashed as soon as the "new game" loading ended.
I completely removed catalyst, and installed the open source xf86-video-ati driver (radeon) instead. Now I was able to start the game and play it with no issues whatsoever.
Keep in mind that the hardware setup didn't change at all, switching the video driver made the trick.
More info: http://wiki.archlinux.org/index.php/ATI
If someone is willing to do the same, I must warn that there are some cascading-dependancies that will require a lot of packages to be removed alongside catalyst (for instance, the whole kde* stuff depends on catalyst), so back up a list of those packages before, to be reinstalled later.
One last thing, upon switching to radeon, I'm getting a lot less FPS on glxgears, but they're still enough to play without noticeable lag.