Here's my perspective on the issue:

Naturally, there are some circumstances where it would be very helpful (nay, vital) to have instant communication with the ground while in flight. Some examples include:
- sick family members
- diverted flights
- real emergencies

The problem, as I see it, is that if unlimited cell phone use were allowed, too many people would use their phones simply to pass the time. We all have heard the types of pointless conversations that take place before departure, and as soon as you turn off the active runway. They often go something like this (and yes, I'm guilty of them as well):

- Hi, [insert name of spouse/partner]........... I just got on the plane........ We are leaving in half an hour......... I hope it's not too bumpy......... Remind Joey to eat his vegetables while I'm gone......... Ok, bye honey.
OR
- Hi, [insert name of spouse/partner]........... I'm on the plane.......... We just landed........ Now we're taxiing...... Damn, we have to wait for a gate.......... How are the kids?......... Still waiting.........What shall we do about dinner?........

Now imagine these types of meaningless conversations for HOURS while in flight!

Heck, on long-haul flights, somebody could use a powerport and AC/DC inverter, plug in his/her phone, and talk for 8 hours or more! Would you want to be sitting next to that person?

So, overall, I think that casual cell phone use should not be allowed while in flight. However, I think that airlines might want to consider it as an emergency communication system (in case the radio is down). The challenge would be to prevent people from sneaking their phones and hands-free earpieces out during the flight unless it were an emergency, or sneaking somewhere in the plane to use their phones. I can see it now--the 2005 AA safety briefing video:

"Federal law prohibits the use of cellular telephones in airplane lavatories, in addition to tampering with, disabling, or destroying smoke detectors"