There is no way that Anet would have set all this up with no way to tell how many different PC's signed up rather than how many different email addresses. MAC addresses are the perfect way to do this and it's how I bet they did it. Especially when you consider that the deciding factor on beta access is computer specs.
I signed up on the PC's at work, my gaming rig at home and my laptop (7 in total). All of them have different specs and if any of those get into the beta then I'll be playing on that computer and giving the best feedback I can. That's what beta is all about.