If you have AMD Card(s) and are finding that 3D is not enabled then you can change that by adding the following to your system.cfg file:

r_StereoSupportAMD = 1

and add this to your autoexec.cfg:

r_StereoDevice = 1
r_StereoMode = 1
r_StereoOutput = 4

r_StereoEyeDist = 0.04
r_StereoScreenDist = 1.0
r_StereoStrength = 2
r_StereoNearGeoScale = 0.2

You may like to tweak these last 4 values to suit your idea of what constitutes acceptable depth.

Source: http://forum.iz3d.com/viewtopic.php?t=3290&postdays=0&postorder=asc&start=45
Source: http://3dvision-blog.com/forum/viewtopic.php?f=12&t=1018

If you find this info useful then thank the forum authors above and BeheadedKamikaze for bringing it to my attention

Edit: tweak stereo convergence settings

Hey, I'm in that thread.