CarRentals.com now has a mobile version of their site. Like the standard online website version, the mobile version allows you to type in the appropriate zip code or airport code and then searches many (if not all) of the local car rental agencies in that area (Avis, Hertz, etc).
Select one and then indicate the dates you wish to reserve the vehicle to see all the different car styles and rates. Then, by typing in your name, email address, etc. you can actually make a reservation via your Blackberry without a credit card. I didn’t actually make a reservation but I assume upon making one you receive an email with instructions from the car rental company as to how you should proceed further.