I use old tights as fan filters
Haven't seen dust yet. As for the Antec 900 problem. I'd say put fan filters on all intake fans. If you can also put a filter on the big 20cm one at the top. Because when its switched off, unless you cover it dust will settle on top and usually fall through. As for the PSU positioning, the Antec 900 would be excellent for keeping the PSU cool, that is if it was the standard. Then the PSU would suck air in, but instead it blows air out. If I had a Antec 900 and my PSU warranty ran out I would open up the psu and reverse the fan and fit a dust filter on the outside part. Then it would suck cold air from the bottom and pass it through the PSU components and it would leave the top of the case as hot air. Maybe soon we will see PSU's with fans sucking air inside instead of blowing it out.