Might as well update this a bit.
AIS Thailand provides Blackberry service, and the service books download automatically on even a prepaid SIM card. But probably for only models they sell, click "View All" here: Nokia & BlackBerry
So, although BB mail does not work on a prepaid SIM, the BB Browser does work and even tethering of a PC. The APN if you want to use Opera Mini, or the Google Apps is "wap" without the quotes.
To tether, I think the only differences are the phone number is "*99***2#" and the Extra Initialization Command is different. I thought it was "AT+CGDCONT=2,IP,internet", but looking at the settings I was using for 2 weeks, I have nothing in there. Any username or password works.
By the way, Google Maps will show your location in Thailand by hitting 0, but names are in Thai script. But searches in English worked just fine, although I tried only hotels and shopping centers, not addresses.
Previously I had used 2 normal AIS One-2-Call SIM cards which charged data by the minute, not by quantity. Can't remember the rate, but it was slightly cheaper than an internet shop and more secure since you're using your own laptop computer, or the browser on the phone. Although at EDGE speeds.
But, if you go to a Telewiz store, you can get an AIS One-2-Call netSim. These have extremely good monthly internet rates, again by time and not quantity of data. 20 hours is $3, 50 hours is $6, 100 hours is $10.50, and 250 hours is $14.70. Plus with every package was some extra time. I think 15 hours extra, but didn't write it down. The SIM itself cost $1.50. For voice time, just add regular refill cards.
Usage is very fair. It doesn't seem to round up to a minute for example and just bills the actual time.