I used an iPhone App called MotionX for exactly this purpose. I'm pretty sure it was free, but if not it was certainly inexpensive.
Track with a Google Map overlay, vertical and speed profile recorded etc, are all very interesting in reviewing the flight. As well as the novelty it's helped when discussing with my instructor (post-mortem style) things like the execution of my solo overhead joins.
As for interference, I hear the typical mobile phone interference when on the ground and when near full power. It disappears from my headset completely in the air below 2400 RPM and either way it's not too disturbing, albeit if it was a passengers phone I would have them turn it off.
Now I have my PPL, I plan to upgrade to Skydemon and an IPAD.
Ian