I used a paintbrush for my gpu fan but I have a Zalman massive copper cpu heat sink and fan together and the dust had made a carpet behind the fan and it was a pita to get to, the hoover didn't do it and nor did the paint brush. Luckily I had some pastuer pipettes from work, they are 2 ml plastic pipettes so I cut the end of one to make it "sharp" and hooked the dust out with that. Cos I need to take the gfx card and sound card out I ended up giving most of the rest of the case a good clean also.
Next time I buy a case though I'll get something thats dust free, however I know a web site that does very fine nylon mesh, down to about 80um, I could buy a metre squared for < ?15 and cut it up and stick it over the fan in ports, that would prolly do the same job ...
Sax.