In the three-wheeled taxi scene, when one of the little taxis flies and crashes into a building, on a wall in the lower left of the screen, under the highway, you can see writing that says: "Hi Luc Besson we are waiting for you".
No wires or CG work were used in the fights.
More than Luc Besson, the crew also seems to like Steven Spielberg. During the alley chase, right after Tony Jaa has performed some break-dance-looking moves on a table, he jumps off and runs away, while the text "Hi Speilberg let do it together" can be read on the wall in the background.
Tony Jaa trained extensively in the ancient form of Muay Boran (the predecessor to Muay Thai) for four years in preparation for Ong-bak (2003).
The style of Muay boran Tony uses is, according to his handwrap, is Muay Korat. Boxers of the Korat style wrap their tightrope up until their elbow.
Ajarn Yodthong (a great master of muay thai) makes an uncredited guest spot role in this movie. He is the man selling cigarettes. This can be seen when Ting fights in "The Pub" for the second time.
Prior to the film's release in the western world, action hero Steven Seagal was so impressed by it that he planned to release the film through his production company with newly-shot scenes featuring himself as the teacher of Tony Jaa's character.