I cleaned (dusted inside) a 40gb. The only major parts to the process are removing the case and the blu ray drive. The case is pretty simple to get off, you slide off the top cover then unscrew the top half of the ps3, and the blu ray drive was too..just be careful not to snap the thingy the drive connects to the board with. I actually found screwing the case back on afterwards the trickiest part, as it fits pretty tightly around the lower half of the PS3 and you'll have to force it on a bit.
You could remove the motherboard if you're confident in taking this sort of tech apart, but it's a hassle and not worth the slightly extra coverage you'll get for the dusting (and once you've gone to that hassle you might as well replace the thermal paste which will be a lot more effective in delaying the fans kicking in).
Use compressed air for the cleaning. Don't expect it to fix the fan issue, just delay them going into overdrive for an hour or so.
