http://www.fluther.com/disc/658/do-cell-phones-really-interfere-with-airplane-instruments/
Already answered. Good question though.
According to MythBusters, “modern planes are well-shielded enough to not be affected.”
Great question, even though it’s been asked already. I actually have an old cell phone that won’t turn off sometimes, and I have felt a little guilty on planes, hoping it doesn’t matter.