You can get at least one decent machining job done on most disk brakes...ie: unless they're worn right down or too damaged to do so.
It's not the o/a thickness that matters but the individual wall thickness on the vented ones...it can't be below a certain amount or you risk warping them.
As for solid disks (most rears) you can machine them quite a bit...
There are alot of good aftermarket disk rotors to replace most OEM types that are priced very well, I recently replaced the front 2 on my Calibra after just over 80,000km instead of machining them, because the new ones cost only $80 each, it cost more than machining them (around $20-30each) but I prefered to get new ones in there...