Sparramc has a video of how he is using an RC servo motor to operate a pneumatic switch.
I think we are all craving a really good way to control pneumatics with a NXT. It would really add a whole new dimension to our robots. The way that I do it is rather than using a servo as seen in the video, I use a medium PF motor. Then I control it using the HiTechnic IRLink. I have tons of luck with it, but one drawback is that I cannot stop the motor in the center position as sparramc has done. Then again, I haven't found the need to do so. Check out an older video of mine. You can see how I did it at about 0:38 into the video.
I am currently using four of these switches to operate the grippers on my Flexpickers. The bad thing in my case is that each robot is run by its own NXT and I had lots of problems because I only had two IRLinks sensors to operate four grippers. But after I ordered two more, things went very smoothly. Just turn the motor on for about one quarter of a second, and it works great.