The AIS blackberry service is 650 baht per month and includes unlimited service.

You simply go to Telewiz or AIS and with your Post Paid account register for Blackberry, they will require the IMEI and PIN from the Blackberry for this.

Is there any pre-pay service available so that I can just swap sim cards on mine when I'm in the country?

Not with AIS, has to be post paid.

Not with AIS, has to be post paid.

You can tell when you have the Blackberry subscription as the word "edge" changes to upper case "EDGE"

EDGE means that your phone has detected the data network and is fully logged in and able to transfer and is able to transfer data packets.

edge means that your phone has detected the data network and is not logged into the data network and cannot make use of any data services.