Problem: I just bought Portal for Linux. When I start the game on my AMD64 laptop with Debian testing, I only see black objects, and a few light stripes in between. Everything else works, I can hear sound, I can interact with objects, and I can look and around, in which case the stripes also move in the right directions, so they seem to be speckles or reflections rendered on objects, and only the textures are missing.

Solution: Searching the Steam forums resulted in nothing (who would have guessed), but this forum post suggested to update Mesa to version 9.2 and install libtxc-dxtn or libtxc-dxtn-s2tc0. This packages were not installed on my system, and the description for the package says that it is used for texture compression, so it seems to be related. So I first tried to install the i386 version:

aptitude install libtxc-dxtn-s2tc0 libtxc-dxtn-s2tc0:i386

After restarting Portal, the problem was gone, so I refrained from updating my Mesa :-)

Before and after images (probably Copyright by Valve, but I consider this to be fair use):