I just watched this review of the HTC HD2:
BBC News - A tour of the new HTC HD2 handset

Dunno if it works outside the UK, if not, the HTC HD2 has a feature which increases the volume of notifications for incoming calls based on the amount of ambient light it detects - so if the phone's in your pocket, it'll crank the volume up. I think this is actually a really good idea, as I regularly don't hear my calls with the phone in my jacket pocket where I also don't feel the vibration, even with the volume at max because my work environment is busy. Ideally it'd also decrease the volume back once it realises you've pulled it out.

I'm not sure how this would work as I've not really started messing with the pre myself yet, but from previous experience I know this should be possible.