Otterbox? Comes with a belt clip that could be adapted...
avoid the otterbox! its a nice case but not fully sealed from the elements, the Griffin Survivor case is nearly fully sealed and its belt clip could easily be adapted to mounted on your stem
http://images.icnetwork.co.uk/upl/birmm ... 210791.jpg
I've managed to drop mine and run it over where I hadn't clipped it back onto my camel back chest strap properly and it was fine once I had clean the muck off the screen
If you only want a case to protect the phone with occassional use while you ride then the griffin is ideal, you can still run the gps tracker apps and if want you can still hear your tunes while you ride
only other advise I would give that would apply to any case really would be to stick another screen protector on what ever case you use, that way if you rub the screen with a mucky finger and scratch it you can just peel it off later and replace it where as if you scratch the cases screen your screwed!