Prospero Technologies:

"Data available as XML - Prospero can already deliver discussion data, message data, polls, and visitor lists as XML, the underlying plumbing of .NET. Registration integration experience - Prospero has vast experience integrating with third party registration systems and expects few difficulties integrating with Microsoft's well-documented Passport registration system (part of the Hailstorm initiative of .NET)."

Prospero is the ASP that powers message boards at sites such as Amazon.com. Its competitor Web Crossing does XML-RPC.