Why don't older Macbooks support multi-touch? Why don't older Macbooks (Macbooks with a physical button on them, I think up until 2008) support multi-touch when upgraded to an operating system that supports it? 

They are able to distinguish two fingers opposed to one for scrolling and secondary click along with also being able to identify more than 2 fingers. Is there a hardware limitation of some sort, or is it just restricted for some other reason?
Hardware limitations.  Note that Mountain Lion's supported models are for at least partially multitouch-capable trackpads; ML doesn't require you to use, say, a 4-finger swipe.

I haven't used any of my Magic Trackpads with 10.8 yet, but it should be fine.