AFAIK, scalability has never been a problem with HD size. Even when the mobo controllers of yesteryear were limited to addressing a max of 128GB, there were HDs way bigger than that and pci cards to get around the limitation. And of course, shortly after that, the onboard controllers were updated anyways.
Seeins how the new macs use the SATA II interface, and Leopard is fully 64bit, I don't think we will run into any HD size or addressing limitations, at least not for a long, long time to come........