There has also been reports that changing sv_friction from 4 to 6 has helped alleviate the problem...
You might need to use sv_cheats 1 first though...

It's not the resemblance to reality. I had the same problem with Wolfenstein 3D (which is a very old game in FPS terms) and Halo but not with other games such as Doom 1/2, Quake, Unreal, Return to Castle Woflenstein, Rise of the Triad (lol), Deus Ex 2, Call of Duty, etc.

I think the problem lies much deeper than realism.

Well call me strange but I have been getting motion sickness playing fps's scince wolfenstien 3D!

Isnt it just a combination of a low refresh rate (say 60hz) together with playing games for a long time (say 3 hours). I always try keeping my refresh rate above or at 70hz and stop every hour when im planning to play for some time. Headache is the thing which could deliver the motion sickness.

I get motion sickness from playing UT2004 after about 30mins..i think i may sit a little too close to my screen, but reducing the view bob really helps..

I experienced nausea after playiing the airboat level for less than fifteen minutes, two different times, and I run my refresh rate at 75, max for my monitor. Didn't have many problems with it since that level though, and it now seems to have gone away totally.

The motion sickness problem can easily be remedied with a third person view point. Find one similar to the 3rd person camera used in farcry