FM Tuner support is there. I'm not sure how much youv'e looked at the source code or the wiki but the whole concept of Open Mobile is a very modular design. Plugins implement interfaces that match their function.
The A/V department is taken care of by the IAVPlugin and ITunedContent interfaces. We already have a media player plugin which is based on DirectShow and can play almost every type of media.
I had plans to do a plugin for the DirectedHD and Visteon unit with mitch's cable but haven't worked out 100% of the details for how I want to implement stream management. One option is to let the tuned content plugins manage the streams completely, handling pausing, buffering, seeking and routing. The other option is to have open mobile manage all this with the assumption that all tuned content plugins will require this functionality. Of course I haven't thought about this in awhile so there could be an option 3 i'm forgetting about.