I don't know about East Coast events, but as far as West Coast- Tribal Fest in Sebastopol, Calif. in May is the biggest, so probably the most ATS by default; also that's near San Francisco, so it would get the most ATS runoff. I don't know of any festivals that specialize in ATS, unfortunately.

The other festivals out here that would have ATS are Cues & Tattoos in Seattle in February, Tribal Caravan near Los Angeles in April, Tribal Rendezvous in Idylwild (2 hours from Los Angeles in the mountains) in July.

as Margaret said,Tribal Fest is definitely the biggest and most well known of the tribal festivals!...but its not known for sticking with strict ATS or even improv in general. you tend to get a very wide variety of ATS, improv, folkloric, fusion, goth, theatre and other experimental bellydance forms at Tribal Fest (which is great IMHO)
link: www.blacksheepbellydance.com/tf8...html

I dont know much about Tribal Caravan, but i'm pretty sure it's along the same lines at Tribal Fest.
link: www.tribalcaravan.com/index.html

Cues and Tattoos however, i do know is an ALL IMPROV festival. not necessarily ATS, but improv.
this past Feb was the first ever Cues and Tattoos and i know next year is in the works as well.
link: www.troupehipnotica.com/cues/index.html
it's still a baby event, but i have a feeling it won't stay that way for long :)

i am not familiar with any Midwest or East Coast events, since i live on the West Coast...hopefully you'll get some more replies :)