Some great posts scattered about here:

http://www.flyertalk.com/forum/searc...ternoon-tea%22

http://www.flyertalk.com/forum/searc...=%22Dim-Sum%22

This place seems quite popular for dim sum: Tim Ho Wan, Terminal 21 shopping mall (Asoke/BTS and Sukhumvit/MRT):

http://pantip.com/topic/34081333

I understand that it is a "chain". They must have promotions on certain days as sometimes there is a huge line, while other days there is no wait.

http://bk.asia-city.com/restaurants/...urants-bangkok

http://bk.asia-city.com/restaurants/...-afternoon-tea