Compressed air is the way to go. Be thorough, and it's fine. I've always cleaned computers at my shop this way. Sometimes I remove the fan first, but unless it's really bad, I don't bother.
I've been quite pleased with my Metro ED500 DataVac, which, despite the name, is a motorized air blower rather than a vacuum. Puts out an impressively high pressure stream of air continuously, good for dusting anything that won't blow away and terrifying cats. Though when we went to clean my SO's PC which hadn't been opened up in 4 years, some of the dust had compacted down into semi-solid form that required even more extreme measures.