have browser, will travel
It appears the official SOAP site for UserLand is at http://www.xmlrpc.com/soap/ rather than http://soap.userland.com/.
DaveNet: An end to the Uber-Operating System.
From the spec: "In SOAP, the mechanism used for all communication is HTTP."
Still, it should be pretty easy to embed a minimal HTTP server into any app to support SOAP. Problem is, two apps can't listen on the same port of the same IP address. Check out http://www.roepcke.com/xmlrpc/ for an archived discussion about this issue. (most of the graphic files are missing, but that's okay, you don't need them to read the discussion)
IMHO, SOAP should be a system-wide daemon/service that apps can register with when loaded. This way, it can run on one well known port, perhaps in addition to port 80 (HTTP), and all remote functionality will be accessible from that one port.