Well, I created some WCF services that act as REST resources, returning geometry objects. I figured if the existing JavaScript, Flash and Silverlight APIs can access this, maybe ArcMap can, too.
But, I used Fiddler to examine the traffic, and it looks like ArcMap only attempts to communicate using the SOAP endpoints. It would be possible to implement that, too. Any idea of ESRI has made public the SOAP API? I don't think so, yet.