Steam or any contact with moisture can damage your iPhone, so it is never a good idea to take your phone into the shower. If the phone is still under guarantee, you could break the terms of the guarantee by doing this.

Even very humid weather can have a bad effect on your iPhone, as can sweat, so you should avoid keeping it too close to you when hot or exercising, for example (I always wonder how anyone can have a phone conversation when working out anyway - it must be difficult!). Keep it covered when out in the rain, and if you happen to spill a drink on it, you should clean and dry it straight away. It is much better, as well as being much better manners, not to have your phone out when you are eating or drinking.

You can find more ideas for protecting your iPhone, as well as an explanation of how water and steam can damage it, here.