Looking for a bicycle community in the Southwest US similar to Philadelphia/Seattle style
According to the League of American Bicyclists, the following communities in the Southwest USA have gold level like Seattle does (no community in the Southwest has Platinum, and none in the USA have Diamond):


Carbondale, Colorado
Durango, Colorado
Park City, Utah*
Santa Cruz, California
Scottsdale, Arizona
Steamboat Springs, Colorado* 
Tempe, Arizona
Tuscon, Arizona


*not sure if this is Southwest

There are several communities in Southern California, Arizona, and New Mexico on the Silver list (as Philadelphia is), although I'm not sure if that standard is high enough to be considered a bicycling community.  Out of the four US communities I have visited, three are listed Platinum and one is listed Silver, and I must say that the standards to reach Platinum appear not to be particularly high and the one Silver community I've listed has hardly any bike trails, so I would not necessarily consider that towns on the Silver and Bronze lists are bicycling communities.

My background/reference: I'm from The Netherlands living in Germany, so my scale may be calibrated differently.  I have previously lived in Sweden, England, Canada (Toronto), and have in the USA professionally visited Fort Collins, Boulder, Madison (multiple times), and many times visited Iowa City (I was cycling on each of those visits).