Source: PlanetHalfLife

People are still experiencing motion sickness while playing Half-Life 2, mainly when operating a vehicle in the game. Still no real fix for the problem, but over at the HalfLife2.net forums some more information from Valve has been posted. Here some words:

We've been rigorously playing and testing Half-Life 2, Counterstrike Source and Half-Life Source for a long time now - and we've found nothing to suggest that the FOV change is a significant factor in causing motion sickness. We have, however, put a great deal of work and attention into reducing the motion sickness that can be experienced in the vehicles in Half-Life 2. During our early playtests, many of us were experiencing motion sickness from driving the buggy and the airboat - especially the airboat. We've done a lot of work on tuning the experience to reduce any ill effects - especially looking at how we manage the players head/view in relationship to the movement of the vehicles. Interestingly, for all of the vehicle sequences we revert back to FOV 90 so that you have more peripheral vision which is helpful when moving at these faster speeds.

The full post can be found here. The best thing to do at the moment is to just take several brakes from playing, especially when playing with the vehicles.