Comp and Cell Phone?

The reason phones aren't allowed to be in "transmit" mode during flight is that the radio frequencies used by cell phones COULD interfere with the electronic navigation equipment in the airplane. It is not because of the magnetic compes. Your phone will not affect your comp, but metallic or ferrous objects could...although this will not be a permanent effect it could interfere with your readings. Make sure that when you use the comp it is a few feet from things like big belt buckles or other metal objects. For instance, one things mountaineers take into account is the effect of holding an ice axe while taking a comp reading. I always stick mine in the snow and take a step away if possible. The effect this has on the comp reading is usually pretty small, but no need to take chances unnecessarily. As for using an iPhone compass app, I would stay away from it. I'm not exactly sure what mechanism is used by the app, but a real compass is much more reliable and even if it is raining, your phone is crushed, battery dies, it is too cold for it to work, etc., a real compass will almost always work.