Originally Posted by Onganju
Considering it's roots go back further than many of the widely accepted TMA's, yes. However many people don't even look that far into a Martial Art. Hell, most are more concerned about if they get belts or not and how fast they can get them. I would realistically consider modern Muay Thai as a more sportative Martial Art, just like Boxing, Wrestling, Judo or Submission Wrestling.
Would you consider Muay Thai a TMA if you learn it in Thailand? The only reason I ask is because it is a lot more spiritual and there's more than just the competitions in Thailand.
But ya, there are some really awesome TMAs, just look at internal arts such as Tai Chi, Qi Gong and Yoga (not power yoga, but other schools such as Hatha Yoga).