I use something a little bit different, essentially it's a 10lb CO2 fire extinguisher with a regulator on it. I had it built to take along on my offroad excursions as a portable "air" source. Works great for tires and even pneumatic impact wrenches.
A local fire extinguisher company assembled it for me after I showed them an ad in an offroad magazine. It cost me $125 US (versus $300+ for the commercial model) complete, filled and ready to go. A refill costs about $12 US and lasts a surprisingly long time.
Just pop an airgun on the hose and it will clean the vast majority of gunk out of a pc. The airgun is easily controllable, anything from a gentle puff to a strong blast. Being CO2 means that its clean, but blast too long in one area and it may get really cold, leading to condensation.
Also, when I clean a fan I'll usually employ a small screwdriver to keep the fan from spinning and potentially acting as a generator.
Oh yeah, I always do this outside. Some of the computers that I've worked on for friends and co-workers have created quite the dust storm