UPDATE: I have now got this working all on the Raspberry Pi without needing the ipad -> see this post
Getting live TV on the Raspberry Pi has eluded my so far, I have tried various ways and nothing has worked but that was until the Raspberry Pi foundation released the mpeg-2 decoder. The HDHomerun uses mpeg-2 so thought there would be a good chance it should work on the Pi with XBMC.
So using the Raspbmc build of XBMC I added the HDHomerun video stream as a source in XBMC and to control the HDHomerun I used an app I have previously tested on the iPad called TV Streamer. The iOS app enables scanning of the channels on the HDHomerun and controlling the tuning. When you select a channel on the iPad XBMC can then pickup the stream and play it.
To get XBMC setup with a HDHomerun you need to go to videos and add a video source:
Select a HDHomerun Device:
Here you can see the HDHomerun tuners available on your network, I selected the first one:
Then to play live TV via the HDHomerun from the videos selection you can select your tuner:
Then before you select the stream you go the TV Streamer app on the iPad and pick your channel:
Select the stream and the channel should play:
For this to work you need to have the mpeg-2 decoder option install (see this post for details). I am sure there are other ways to go this without using the iPad but it works for me and I now have live TV on my Raspberry Pi.