Casejunkiez have posted an article titled 'The Delta Filter Experiment'. Heres a little quote:

    While the intake and exhaust fans provide fresh airflow into your case, the CPU fan is causing turbulence inside, sucking that air in over your heatsink fins. This is a prime target for the majority of the dust entering your PC, and if you have a high RPM fan on your cooler, it makes things that much worse. Pets don't help the situation much either, especially when you have a little black cat that likes to leave fur everywhere. :)