Here's how UPNP currently works. Note that this is beta firmware, things will likely change.
1) On the RC9800i you configure a UPNP media PLAYER (not PC server). I think this will work with any UPNP player, but I don't have a manual so I don't know.
2) After configuring the media player the remote registers the UPNP server (AKA your PC) that the player connects to.
3) At this point you can now browse the media files on your PC.
4) If the media player is powered OFF, you can still browse files, display photos on the remote and preview MP3 files (note that I have not been able to get MP3 previews working, but it was demonstrated at CES).
5) If the media player is ON you can preview files and send a command to the media player that causes it to load up the file and play/show it on your home theater. And fast forware, rewind, skip to next file etc. But I cannot currently test this as the Philips media player has not had its firmware upgraded to support it. Because of this, I suspect that this feature will only work with Philips Streamium players, unless other manufacturers decide to support it.
6) I don't know about internet radio stations. The Streamium media player does that directly instead of through a PC, and since the RC9800i only seems to display items from the PC, no stations appear in my setup.
I hope to be able to clear some of this up when everything is finally released and I get finished software and a manual.