The only way I can see it work, is to show scrollbar on every mouse movement. But than I don't see any benefit of hiding it in first place.
However I agree with you that we will see how it works out, when people start using it.
The worst case scenario is probably increased bounce rate on some websites, decreased conversion rate, and some confusion for users.
It's like the "users don't scroll" myth. If a user is reading to the bottom of the page he will flick it up to see if there's more below. And it's the designer's job to make sure that the right content is findable above or below the fold.
FYI, the scrollbars are always visible on my MBP 2011 using just a trackpad. I love it.
If they did disappear, it's merely a quick two-finger touch of the trackpad to see where I'm at and how much is left.
As for whether or not it's a good idea, in practical experience it seems to be fine.
Disclaimer: I'm just a happy SizeUp user =).