Found another "swapping for art" site called Let's Swap. It's got a nice, clean design and an intuitive user interface. I'm jealous.
They do one swap at a time. For example, right now the Featured Swap is Mike Perry's Public Works collection. There are 5 available. You can submit a trade offer and then if Mike picks your swap - he'll trade you one of the five prints that are up for grabs.
Get in there and try it out. Shoot me an email if you get a trade accepted!