(Shameless advertising but I've recently put up a forum for all kinds of mail artists, so anyone interested in discussing ATCs, decos, altered books and such are welcome to join.
I know it's mainly decos as the name suggests but it'd be great to get some ATC people there, too. Link in my sig - Deco Swappers.)
Flickr is also good for ATC trades, there are a lot of ATC groups there. Some of the groups might arrange organized swaps but I guess most of them are meant for showing off your art so that you can find people interested in private trades.
Lol, I applied to Illustrated ATCs once and didn't get in because I linked my whole DA gallery instead of sending a few pics. I don't think it was said anywhere that you were only allowed to link a certain number of pics (seven or so, I don't remember)... I only got to know this when they e-mailed to inform me that my application was turned down. :'D I could've applied again after six months or so but I never did, I do my trades on Flickr, DA and here. But they sure do have some fantastic art on Illustrated ATCs, I sometimes go there just to look at the beautiful artwork.