There have been numerous threads on this topic. Do a search.
Short answer is yes, they can be if the plane's electronics were not specifically designed to handle it (cable shielding, etc.) Ever put a blackberry next to a speaker phone and listen (radiated suspectiblity). Now imagine a pilot trying to listen to the tower doing critical final manuevering to landing during an IFR approach.
If everyone put their ipad on airplane mode, they can use them today midflight. If they all have 3g ipads and have the 3g phone service on, that could be a problem.