I've been looking for a similar solution for my iPhone too.
I've looked at three or four different waterproof/protective cases. My problem is that I don't want to have to switch cases for rainy days and non-rainy...or for when I'm not even on the bike at all. The iPhone being my main communication for work and emails and the like, I need it all the time. So finding the right waterproof case means finding an all-day, any-day solution.
That aside, I've found the following three cases of which I've yet to decide.
The BioLogic Ree Charge Case
- designed more-so for cycling it has a battery in it that will give me more time for the all-day rides...since the GPS really eats up the battery. But, with the right charger mount, this isn't really necessary... Cool feature for a cyclist though is that you can buy a hub kit for your bicycle that will charge the case's battery while you're riding. Another drawback is that it appears the case might be significantly larger than the iPhone itself.
The LifeProof Case
- I think this might be my choice simply for the low profile-ness of the case. Saying it only extends about 1/16 of an inch from center all the way around. So, pretty slim.
The Concord KeyStone Case
- this is the cheapest I found. It may be a good option for having at the ready when the weather turns bad and then not having your iphone in a case at all other times. Unfortunately, it's kinda ugly to me. But for $40, the functionality outweighs the looks.
For mounting it, my only considered option is the RAM Ball Mount for the Clutch Reservoir Plate
- I like the offset ball idea as I've heard the centered ball mount can impair the left mirror view. This option will also need a mount for the iPhone itself...considering the waterproof case's size, this
might be the only option.
Anyway, I'd love to see what you come up with if you stick with the iPhone for your GPS Solution. These are just some things I've found in my recent searching.