but wouldn't it make sense for Apple to pay to do the testing? Being able to say "The iPhone is the only phone you can use during your whole flight" would be worth quite a bit of money to them.
Delta... the only airline you can use your iPhone 4S from takeoff to landing...
EDIT: Almost forgot: After the testing is complete, you have to pay to have a test report written up and published, and then you have to pay for the FAA certification process. Probably not cheap.