The initial query from Keygrip seems to refer to something like the Nokia 9210i
The mobile has a "Flight Mode" that turns off all eternal transmissions and signals - but allows the 'PDA' part of the handset to be used, in 'flip-open' mode.
The office function, spreadsheet and voice memo can be used without any external electronic signal transmission - but to any third party, it would appear that the 'phone is still 'on', so the perception is created....
I agree with G-Fox tho' - Iv'e left my mobile on (by mistake) in my flight bag on the back seat of a small GA airplane before, and there is nothing more anoying than speaking to the tower for clearance with the interference in your headphones - you can always hear other traffic with cell-phones left on in the general radio calls as well...
Of interest, I see in some US flight mags (I'm in darkest Africa) that you can buy mobile handset adapters for the intercom / headset system - I know that here it is illegal to use a cell-phone in GA aircraft, but I do not know the US law on the subject - interesting point tho' given the interference with RX comms.
I must admit, given all the above, that my cell came in useful on a night flight when I had complete electrical failure in a C210 - called the tower and got all clearances by phone, including nice bright runway lights...
R