There's a cool web site that attempts to point out decent espresso places in North America: http://www.espressomap.com.
It certainly doesn't list ALL the good shops in N. America, but the shops that are listed are at least a good bet.
Cheers,
John
P.S.: I sure wish I'd known about this forum before we visited the U.K. last summer. We were bummed to not have any idea where to look for a good espresso.