I was thinking of writing something like this to REPLACE my remote control (basically send up/down/left/right/etc via home WiFI network). Just about to get an Android phone that is capable enough.
Android Remotes for Mediaportal / Windows PC
Gmote - Gmote > Android Remote
Gpad - gPad remote for android - HOME
GremotePro - GRemote - the best way to control PC using PDA (Home)
Remote Droid - http://remotedroid.net/
iPimp
I was thinking the same thing about an iPhone app and started writing one (including a process plugin for MP).