Olivier wrote:BKK - Bangkok? YES

DAD - Dallas ? NO
HKG - Hong Kong ? YES

SGN - Singapour? NO
Olivier wrote:BTW, do you know how the codes are chosen? Is there an office which say "your code will be, well... EBT!" ?

They're 3-letter codes from IATA (International Air Transport Association) - "Location Identifiers", in aviation-speak, "luggage-tag codes", in jargon. There's no airport with EBT code, though
There are also 4-letter codes by ICAO (International Civil Aviation Organization). Here's where I've been:
EGGL
EGKK
FNLU
KJFK
LFPG
LIPZ
LIRF
LOWW
LPFR
LPHR
LPLA
LPPT
LPSJ
LSZH
VHHH
VTBD
VVDN
VVNB
VVTS
Now try find what those mean!

(I know how the codes are formed, though. If you want an entertaining explanation, read
http://www.skygod.com/asstd/abc.html ).
Now, for fun, here are some names for some of these airports:
4 de Fevereiro
Charles de Gaulle
Chek Lap Kok
Don Muang International
Faial
Fiumicino
Gatwick
Heathrow
John F. Kennedy
Kai Tak
Kloten
Lajes
Leonardo Da Vinci
Marco Polo
Noibai
Portela
Schwechat
Tan Son Nhat
Tessera
You're already familiar with some of them, but I bet other people will be puzzled - even NOW1K (by the way, is this any airspace code?

) and whoever might have worked in a travel agency.
EDIT: added another one I had forgotten. Still not sure about 3 others...