I know 2 people with gaming microATXs, one from alien ware, and was the best-of-the-best about 5 years ago. Was great then, these days, heat and melting is an uberproblem (no doubt due to clogged upedness). Anyway, the guy bought an alienware, so you can imagine he doesn't open the bonnet very often.
The other guy build one from scratch and the build is of the same calibre/generation as the alienware, but build about 2 years later, so was reasonably cheap at the time. Again, he had to be really careful with wiring and so on, and generally always keeps an eye on temps and things. I don't think he has ever had major worries, but things do run hot in his computer.
The trouble is, todays GPUs, CPU etc. run hot and take up much more space. Hence, I can imagine it becoming more and more of a problem as the technology improves.
I'd say its doable, but something you want do research well before you attempt
(oh, and don't forget about lack of upgradability)
