I searched but all I find are iPhone apps that simulate a mouse, provide kbd input to the HTPC etc.

What I want is a full-blown iPhone-HTPC interface. Something that will let me control the HTPC WITHOUT turning on the TV. Lets say I want play a song stored on the HTPC on my stereo, I wanna list my tunes on the iPhone, press PLAY and presto. Don't even have to go through the TV.

Seen anything like that for either the MAC or PC platform?

Any of the HTPC front-ends support an iPhone interface?

What software are you using on your htpc? If all you want to be able to do is control your tunes, there's the great free app from apple: remote (if you're using iTunes) But if you're using MCE or something I'm pretty sure there are a few solutions in the appstore.
I use my iPhone to control VLC, PLex and iTunes. Works great, and no tv involved of course (except when watching video:) )

You do have some options: iTunes will allow you to control it from the iPhone using the Apple remote. I think MainLobby has an iPhone module but its based on the web interface and not an app.