Pretty much all the shops here sell more than one brand, but most of them only sell one of the "big" American brands - Specialized, Trek, Cannondale. My LBS sells Specialized, Bianchi and Serotta, and also carry Time and a probably a few others. There's also quite a few stores that carry Surly in addition to their main big brand (Surly is local, after all).

When I was in high school the shop I worked at sold Cannondale, Trek and Schwinn, with about equal floor space for each, plus a plethora of other brands via special order. I guess those days are gone.