I was looking for occasional-use mobile broadband, which didn't involve a monthly fee or contract, as I didn't want to pay every month for something I would only be using once or twice a year. After taking prices and reviews into consideration, Virgin Mobile (via the Sprint/Nextel network) and Verizon (via their own) became the top choices. Based on the kind of use I'd be making of it, coverage maps, and past experience with Sprint (granted, it's been a few years) and Verizon, I decided to go with the latter.
I started by trying to deal with a local store earlier today, but no one there was trained on dealing with their occasional use prepaid broadband service (known as DayPass). In place of real information, misinformation was offered instead. I knew from the website that both built-in and add-on EVDO modems would work, but according to one person there only the former was an option. The others I dealt with just didn't know.
I came back home and called up the number given on the DayPass page for placing an order. It took a while, and involved 2 call transfers, but I was finally able to place an order for the hardware. Apparently, once the hardware and software are installed, I activate it through the computer (that's the ideal, anyway) and then pay for time as I need it. The card should arrive later this week, and I'll set it up and pay for a session just to make sure that everything works.
The process could have been easier, but at least the result is what I was after.