No. You can fit MicroATX and ATX boards inside an ATX case. A microATX case though is much shorter and has fewer expansion slots. The only way to fit an ATX in there is to cut though the bottom of the case which would have the bottom hanging out. Since that's usually where the IDE or SATA ports are you're gonna have to run those wires back up through the case. It's really not a good idea, but hey it's your crap so do what you want with it.
You sure about prices re uATX vs ATX? Where I live (Oz) uATX boards are usually cheaper than ATX...
I agree there are some nice small uATX cases - pity they all have low watt power supplies which limits your CPU/GPU choice.
I've got a nice small uATX rig at home I use for work (programming), got an E2180 @ 3GHZ, 4GB DDR2 + a 8600GT. It's a nice machine for normal use but too light weight for gaming. The limiting factor for me is it's 250w power supply. It's nigh on impossible to get any 3rd party power supplies for these uATX cases due to their smaller custom size.