From my humble understanding it has to do with the microwave waves that mobiles use to transmit, the same ones that no-one can decide whether they fry your brain or not.
A quick google yielded the following
From
http://www.meteor.ie/misc/health_science_faq.html:
"Aircraft contain a vast array of complex electronic equipment and communications systems. Using a mobile phone in an aircraft could cause interference to these systems because they are not designed to tolerate such interference. It is for this reason that the use of laptop computers, portable stereos and other personal electronic equipment is also prohibited on board.
The credit card mobile phones now provided on some aircraft do not interfere with aircraft systems because they are wired to a special "base station" that has been designed to safely receive and transmit radio signals in the aircraft."
There's also a page full at:
http://www.geekzone.co.nz/content.asp?contentid=800