This subject regularly does the rounds on the rec.aviation.piloting newsgroup (go to
www.deja.com if you want to read it).
Anyway, there are a couple of reasons that are usually given:
1. Mobiles were designed for land use, and the speed at which an aircraft flies together with the large number of cells that the phone can see conspire to "confuse" the cellular network and reduce capacity (allegedly). As a GSM engineer, you are more qualified to comment on this.
2. Electronic equipment for aviation use undergoes certification. It is possible (and I stress the word possible) that a mobile could interfere with equipment on the aircraft. The mere possibility results in a prohibition. Professional airline folk in this forum are more qualified than me to comment on this one.
Interesting, here in the US, the FAA prohibits portable electronic devices from being used during takeoff and landing, but it is the FCC that prohibits mobiles in flight. However, the FCC regs specifically mention certain frequencies and US GSM only phones (operating in the 1900MHz frequency band) are not specifically mentioned in those regs.