Apple's new iTunes 8 has hefty requirements. If you want to watch iTunes HD on a Mac (or a PC), you need at least a 2.0 GHz Core 2 Duo CPU. That means that all Mac minis except for the latest top of the line model are excluded. In fact, Apple's entry level Mac mini being sold now doesn't even meet these requirements. Some older MacBooks, Intel iMacs, and even some MacBook Pros don't meet the requirements either.
Fortunately, I sold my old 2.0 GHz Core Duo MacBook (not Core 2 Duo) last month. I am using a G4 800 iBook for the time being. I'll probably buy a new MacBook in 2009, but I won't rule out buying a new one in 2008 should Apple release a super-duper new model this year.