I was looking for a good usable solution for my iphone since it would a lot handier to carry than a phone, gps, ipod and spot Satellite tracker. I came across this case in my search. It's made by Dahon....a company that makes folding bikes. It's one of their accessories.
It's weatherproof and still allows you to use the phones screen while it's in the case.
Another interesting bit of software to consider is http://www.instamapper.com/
Works in a similar way to the SPOT system, periodically sending out signals and tracking your progress online. Best of all it's free. Only problem is that if you want to run this and your GPS navigation software at the same time, you need to jailbreak your iphone so you can run instamapper in the background. But it all seems to be working well so far.
If you want to be able to use the screen without having to take your gloves off, conductive thread is the answer.