I have an example of every generation of ipod since launch and use most of them every day. I also have 2 iphones and a macbook - Ive never had an issue and dont expect one.
But you need to be aware what you are carrying on your pocket. Li-Ion polymer cells do take some looking after. A large part of the "battery" is taken up with cell management technology to try and prevent thermal events, as they are very destructive when they let go. Thankfully incidents are rare compared to the popualation of units in circulation so the stats are on our side.