The more recent exurban developments of Denver are pretty bad, controlled only by economic implosion. The central city and streetcar suburbs and early postwar suburban developments are very similar to Seattle's, without the water. Downtown is very similar, traffic-wise. Denver doesn't have any real hills like Seattle. Denver's core MUPs are much better than those in Seattle. Denver seems to have about as many good bike lanes as Seattle. What Denver doesn't have are Seattle's drivers, probably the most accomodating drivers in the nation. Denver drivers are not the worst out there but are down the list a bit.

What about a place that doesn't have any of that stuff, but does have a rapidly growing population of bicyclists anyway -- Detroit? What's happening there?

In my opinion, reaching said tipping point in Seattle and Portland has had more to do with social and cultural factors than infrastructure inducements. Although I think infrastructure can play a positive role, we need to guard against simplistic notions about infrastructure leading so easily to more and better ridership. Simplistic beliefs in this regard lead to bad facilities designed by and for novices which ultimately stunt the growth of transportational cycling. We need to apply a little more subtlety and nuance as we move forward.