Lua is not good for coding AI's. It's just not fast enough. Going through Lua on every creature update cycle for every creature would just be a disaster. This is the primary problem with making a Lua binding: people would use it disasterously and then give mangos QQ when it doesn't work well.
There are also a few other problems. For example, if you want to store a timer variable in an AI, it would have to be stored outside of Lua anyway, and passed back and forth between AI calls, only further slowing things down.
Lua's only realistic use would be for GO/item use, gossip scripts, and other such "one-shot" functions. I was considering making an interface that supported these functions. If there is actual interest I might consider giving it a shot.
To code AI's: use C++, use EventAI, or don't.