Here in the US, mobiles can be used until the main door is closed ... and the FAA have just introduced a new relaxation that they can be used right after landing when the aircraft is taxiing. Not sure if all the airlines have implemented this, but Continental announce it once off the runway.
The actual rules (part 91) the FAA lay down is that no electronic devices can be used on an aircraft operated by a air carrier / commercial operator or on a civil aircraft on an IFR flight plan. I guess there must be another exception for game boys or something.
As far as the effective height for cell phone use ... errrrr, text messaging works ok up to about 7000 feet though voice doesn't, so I am ... assured
Wasn't me, honest guv !!!