Re: Trek vs Specialized vs othersSome pro riders actually ride their own bikes except with their sponsor's paint job over it. I don't know if this practice is still commonplace but it's easy to catch if you look closely.
Trek makes fantastic bikes in my opinion. If you're wondering about racers' preference, the bling factor in comparison to the more exotic European brands (i.e., Pinarello, Colnago, Willier) is kind of low. Buying a Trek is kind of like buying a Ford or a Toyota in many people's opinion.
But keep in mind that the income of the average amateur racer is over $100,000 USD (at least it is in Texas), so they can usually afford really fancy stuff and even some of the Cat 5 racers can be seen on $10,000 bikes.
I ride a Gary Fisher which is pretty much the same thing as a Trek, and so far it's a good bike. I also have a Trek that I use to commute and it's about 8 years old and is almost as good as the day I bought it. Probably the only thing I don't like about them is looks. You'll get the "oh, that looks nice," but not the "damn, that is a smoking hot bike," kind of response...the important thing, however, is to let your legs do the talking.
As for the prevalence of the bikes it really just depends on what bike companies sponsor the teams in your state. Over here in Texas, we do see a lot of Specialized and Trek because they sponsor teams here. Also Cervelo is really big here too. Surprisingly, Giant isn't very popular around here even though they seem to be pervasive worldwide.
There's also a controversial practice in the bike industry where big companies will give discounts to bike shops if they exclusively sell a certain product. I know Shimano used to do this and it's one reason why they became so dominant in the U.S. An antitrust suit put a stop to this practice by Shimano and allowed SRAM to become active in the componentry market. I really wouldn't be surprised if this were still practiced clandestinely amongst the big brands, however...