Came across this site and thought it was worth posting as many that read probably use the ATM to take out cash from their country of origin. The site basically gives you an up to the minute rate given by the high street banks here so you can compare and go to the one giving the best rate on a given day.
All you do is put in your home currency to be changed into baht, change the 'currency type' box to TT (ATM rates) from notes and hit the 'show rates' button and it will come up with a list.
Good site, sometimes the difference isn't much but sometimes it is. Depends how much you take out, but it could easily save the charges of doing it if just small amounts and more if bigger amounts. Certainly makes a difference over time though.
http://bankexchangerates.daytodaydata.net/default.aspx 
