By the way: My recumbent bike has even smaller tires (20") but it is way faster. The size of the tires is not the limiting factor in unicycle speed --- it's the fact that you have to keep balance and input power using the same system.
Right, which is why my speed tends to very briefly approach terminal velocity.
I have a fixed-gear mountain bike with 700c x 42mm ("really big") tires, and that doesn't really affect speed much either. The wheels very clearly have a sort of flywheel effect, though: once I'm moving, they hold a lot of momentum. (It's excellent for commuting in snow, BTW.)
Bike Friday (http://www.bikefriday.com/) and Dahon (http://www.dahon.com/) are popular in the U.S.
In case you don't think folders are "real" bikes, note that they are ridden by many serious cyclists (http://www.bikefriday.com/node/4813).