Since many airlines now allow use of phones during taxi in to the gate after landing, I can't imagine it will be much longer before the airlines start allowing phone use during the taxi out (especially given that it can be extremely length taxi times, with ATC holds, WX problems, etc).
I don't see why it is any more dangerous for people to be using their phones during taxi out than it is during taxi in.
I've seen many FAs continuing to use their phones/iPhones/Blackberries after the door closes and the taxi out begins.
My suspicion is that within a few years, the FAA regulations about the use of electronic devices will change. The prohibition on mobile phone/data usage will apply only from the moment the plane enters the active departure runway for takeoff until it exits the arrival runway upon landing.