My wifes 24" 2009 iMac lives in a very dusty environment (office with open windows in a hot Caribbean island) and when I opened it up to change the hard drive last year (upgrading from 500 megs to 1 TB) the dust was not all that bad. I was very surprised. I blew out what dust accumulation there was with an air compressor and its been fine.
I got a 21.5" 2011 iMac for myself and won't be worrying about dust for the time being. I will probably pull off the memory cover every now and then as I understand dust can accumulate on this as the fans draw air up and through the perforations. This will probably give you some idea as to whether dust might be accumulating inside the unit.
I don't see why there should be much difference between the models when it comes to dust collection...