I'm currently working on setting up Meijer and Wal-Mart AS2 solutions, so far, everything is going great, not much problems.
To setup the entire AS2 solution, you just need any computer with the AS2 software setup and configured properly, rules for ports to open in the firewall, NAT for those who decide to put it behind the firewall and that should be about it.
Unixguy, yes, they're just software, not full blown servers. I think whether you need a dedicated box really would depend on your traffic, I am just running the entire thing as a service at the moment.
Soupkid, what problems are you having with listeners? List more info and maybe I could help. I've had some trouble myself at start.