I am really glad that Surferbruce brought up this topic. I, too, am rather perplexed about how you shop for a bike in a LBS. I live near Santa Cruz, CA, which is really a major cycling community. When I decided a little over a year ago that I was going to get a new bike, I went to the five local shops that deal seriously with road bikes. There was nothing for me to try. No one had a 54 cm bike in my price range (up to about $3,000).
So I set off over the hill to San Jose area. Not much better over there, except for a shop that carried Trek and Bianchi. I'm not sure if Trek is really that popular or is it just that its dealers get to have some inventory. Dealers can make deals when they have something to sell. Similarly, since the West Coast distribution center for Bianchi is in Oakland, it's easy to get test ride on a Bianchi.
Since that time, one shop in Santa Cruz has become a major Trek dealer. The shop is full of bikes and they're selling a lot of them. Having a Madone out on the floor to be admired and touched is a great marketing tool for selling cheaper bikes: Even if you are buying a $1,000 Trek, it feels good to buy a bike related to the Madone.
Eventually, I went back to the shop in Santa Clara that originally fitted me. When I had my fitting, I was not as interested in the custom steel frames they built up there. I was hoping for a carbon or ti frame. The more I tried to look at other bikes, the more I realized that I should have a biker built to my specs. Shaw's was committed to making sure that the bike I ordered was going to fit right and ride right. My new bike worked out just as promised. It's perfect.
However, I am still clueless as to how to shop for an off-the-rack bike if there is nothing on the rack.