I'm headed home from Bangkok this evening and was shocked as well when a few days ago I suddenly got hit with a 150 baht transaction fee from a Siam Commercial ATM. I'm not clear if the fee applies to all Thai banks, though, as last night I used a Kasikorn ATM and was NOT charged ANY fee. It could be that the ATM simply hadn't been reprogrammed yet, or perhaps only some banks like Siam and Bangkok Bank are charging the exorbitant fee. Prior to the hike I know that some banks were charging a 20 baht fee and others like Kasikorn and Ayudyah (I think) had no fee.
BTW, this morning's Bangkok Post has an irate letter to the editor complaining about the new fee.