I have a Curve and I've seen a lot of Blackberry users around. These are personal phones and people use anything from Rarz to PDA phone. There are also firm-provided phones but some people avoid it like a plague. The firm also has deals with different service providers that give discounts to their employees.
I saw a few phone stores around work that if you show ID from Bear, JPM, etc, you get 15% off your contract.
In short, people use whatever they want and Blackberry is popular because you have email access to your personal and work account.