I've been doing some research on this subject and have found on ebay a waterproof casing for the iPhone that looks respectably sturdy and comes with mounting hardware for it.
There's also a universal 'zip up' case which doesn't appeal to me but the other looks hard shell and tight. I'm waiting for the iPhone 4 compatible version to arrive. The unit I'm thinking of has specifically designed ports for power etc.
Once I get rid of this infernal blackberry, no offense to those who love them as I too have been a fan for 6 years or so, I'll be custom fabricating a dash mount for the waterproof moto case.
The good stuff is out there! Don't use a zip-loc bag fellas!!