I use my iphone almost all the time. but I have a 3 year old Nokia candy bar that I plan to use as an iphone backup for rugby season. My thinking is that during the season my personal belongings either 1) sit in a hot car all day while I run around, 2) sit on the side of a field with my team and onlookers who are strangers, on a field that could be muddy or rainy, or 3) going with me to tournaments where i am staying in a random hotel or someones floor, and i might not be around my stuff, or i might be dirty/muddy/sweaty.
I tended to do the same with my Treo when i had it. I'd rather be out the cost of a 50.00 nokia because it was stolen or damaged by the elements than my nice shiny expensive phone. This is also why I love GSM, I can carry my expensive phone 99% of the time and swap out to a cheaper one when its not rational to carry the expensive phone.