You can check these links, or search on Google "ham operator license thailand".
This site information was updated 15th Dec 2003, so information most likely has been changed.
http://www.qsl.net/rast/text/Reciprocal ... 15Dec.html
This site refers to documents needed for validation. It seems that if you already have a license from certain countries that Thailand has agreement with, you can operate in Thailand with some limitations or even obtain a Thai radio operator's license. Depends on the situation and whether you are going to be living and/or working here.
http://www.echolink.org/validation_docs.htm
Good luck
-.-. .-.. .. ..-. ..-. Pop's
(not sure kinda rusty on my morse code)