If I could watch my mp recordings anywhere on my phone that would be absolutely amazing!!!
First of all the web app should let you control the mediaportal from the iphone in an easy way, like the OP posted about the XBMC project.
Would be so nice to control which music is playing, change playlists, control the volume etc wherever you are in the house!
You have other applications on the iphone to see recorded tv, movies etc on the iphone, example ORBLive.
I think the Orb Mycast iPhone App would be a great one to model for the MP iPhone remote....that's a pretty easy way to get the UI, but I think you'd need to compile a custom web server to interface with the MP API, as I don't think it's web-service enabled. I suppose you could probably write a .Net library that would interface with the MP API, and hit that library from the .Net app that is serving the pages to your phone...
at some point, you begin mirroring the MP Web Interface application that's already been produced...it seems to make the most sense to just extend this to include things other than TV (which it works very well for).