The only place I've seen them is in Cleveland, Ohio of all places.
It was a "Brazilian" restaurant, though... which are sort of a fad, I think -
the kind of place where they bring around all kinds of barbequed meats as long
as your table marker indicates that you want to eat more.
Even then it was the vodka version (and a terrible, awful, all-vodka
concoction at that).
The first (and last) true Caipirinha I had was when my brother brought home a
bottle of Cachaca and some Brazilian brown sugar cubes for muddling from a
Karate tournament in Brazil...
I don't think they've caught on here yet (at least in the DC area), but
there's certainly potential, as even the Indian restaurant is ordering double
on their mint to keep up with the demand for mojitos (and they make them VERY
good).