The inetmgr tool has been designed and implemented with the ability to run on down-level operating systems in mind, and some basic experiments with installing the binaries on XP have been promising. That said, we haven't made a final decision on what do, but it certainly is a possibility.
Microsoft.Web.Administration currently will not run on down-level operating systems because is has dependencies on functionality only available in Vista and Longhorn Server. We will be trying to figure out a better remoting story for this API in the Longhorn Server timeframe and this may include the ability to run programs from a down-level OS.