Sure. Late 1999, iMacs. Only those with FW can run Tiger. Base model stuck on Panther. 2003 17" iMac. 800Mhz is stuck on Tiger and the 1GHz gets Leopard. Snow Leopard also requires certain video cards for hardware Quicktime or OpenCL, although this is clearly less arbitrary. There are others I'm sure.Really? Do you have an example where the base model wasn't supported but systems with add-ons were supported? That would surprise me.