Strong magnets generate strong EM (electro-magnetic) fields, which is how cell phone signals get to your phone. Inside the phone, there are are things which are EM field susceptible. It would take a special shielded case to allow your iPhone to float safely, and such a case would also do a bang up job of blocking signals from getting to or out of your phone.
So you could theoretically have your phone float and just be a cool iPod touch, I guess.