I have a 2010 Mac Pro and a 2010 iMac. I have not noticed much dust in either machine, and I have not had to blow out that which was there.
I opened a 2007 iMac and replaced the hard drive back in late 2010. That's 3 years of daily usage. It had dust in it for sure, but not nearly as much as I thought it would. I also tore down this 2006 MacBook Pro when I bought it a few months ago from a friend, reapplied thermal paste and cleaned out every inch. It was loaded with dust. Way more than the 2007 iMac I worked with. I think the person who used this lived in a dusty environment or was a smoker.
Yes it was a pain to open the iMac if I were just cleaning it, but after 3 years it wasn't that much in there. A quick use of compressed air through the three fans and vents both in the in-direction and out-direction took care of it.
With the new iMac having just one fan it seems, perhaps the chance of increased dust build up is lessened? There might be a fan over the CPU too for all we know.
Now, comparing this to my wife's current PC, the dust problem is crazy! I think it's the poor design of the case or something, but the vents get loaded up with dust every month or so. It's a mini ITX Lian Li box with a 140mm intake fan. The CPU has a little 60mm fan on it or something, whatever the stock AMD fan is. The PSU has a 120mm exhaust fan taking air out of the case.
Granted the PC is on the floor and an iMac up on a desk, the difference in dust accumulation is still interesting.