For every model from the G3 onwards, there's technically no problem using the proper kind of PC memory (PC66, PC100 or PC133 Dimms and SO-DIMMS), unless you run Apple's latest firmware update which disables non-Apple certified RAM!!!
I don't believe this is a bug and I can't believe Apple would sink so low to validate their ridiculous RAM prices ("well our RAM works with the new update, so it's obviously superior quality.") Apple's going to get hell for this one. This could get some really bad press.