I opened up my mid-2007 iMac recently to replace the SSD and did not notice any dust issue. I think the way these iMacs were designed, minimizes dust issues. Meaning, the enclosure seems to have no easy path for dust to collect and/or enter. For example, my mid-2007 iMac has vents at the top for air intake and air out-take. But, they're angled in such a way that dust can't just fall in through them.
I also have another Mac with similar enclosure, a mid-2010 15" MBP and I didn't notice noticeable dust buildup on it either when I opened her up just now to show you guys. Just small amounts of dust on the fan blades. This is another enclosure where there are no noticeable/obvious gaps for dust to go into, except through the vents in the back where the LCD hinge is located. This is also where the two fans are located to suck in cool air and then blow it out, through the same opening.
As you can see on the photo above, there are only minor dust buildup on the blades, which can be easily removed with a brush and one of those pressurized blower can things. FYI, did not dust the computer for months, if not a year and a half. And, this photo is as is without me dusting it before hand. Nor, would I be that lame to tamper or doctor or lie about it, either, just FYI.
I mean, if you look at the design, there is no where for dust to enter.
This is different when you look at the design of the classic Mac Pro tower, the cMP 5,1, which I have. It's a well ventilated and roomy case where dust can enter. Even though it relies on positive pressure, dust can still blow in from the vents and accumulate inside. But, one would simply periodically dust out the inside once in a while as well and that would solve it.
I wonder if the situation mentioned in that MR article yesterday where there is a class-action lawsuit regarding dust in iMacs are rare circumstances with particular environmental and situational nuances involved in creating what to me seems like abnormal dust bunny accumulation.
I also dont see how it can enter in between the screens.