The question is old yes, but I'd like to add my thoughts to this.
I have been in BKK and southern region recently, and AIS (usually green color package) worked best IMO. A friend of mine bought a TrueMove (red color package) and it was OK too, but signals were not quite good as AIS.
In the Suvarnabhumi airport, go to the airport rail link. Near the 7-eleven, you'll see an AIS stall. They have the micro SIMs (and they didn't cut a mini SIM for me).
Beware that there are some packages with unlimited data for 2 weeks. The speed is terrible (similar to EDGE). AIS was good enough for Google maps and stuff.
You'll find many Wifi hotspots in Pratunam shopping complexes. I didn't really used data until we move to Pataya, Krabi, etc.