Cingular calls their UMTS network "BroadbandConnect."
You can find a list of cities here:
http://www.cingular.com/business/3G_cov_maps_pop
Bear in mind that to take advantage of it, you'd need a device which supports UMTS on the 1900 MHz band. There are a few phones like this already available, but no WM Smartphones that I'm aware of.