Make sure it has big easy to see buttons and a good speaker on it. My folks are gettin old and we've been through quite a few phones trying to find one they can hear well on and see the buttons.
My Mom's 82, she has the least expensive service that ATT/Cingular offers, $20 or 30/mo. Not sure what prepaids cost, she has used it when she was stuck at Detroit airport on her way to visit the grandchildren in Texas, etc.
I don't know if this thread is meant for advice on the best service to choose or the phone you should get her.. in case it is the latter I recommend you get her the motofone F3 its built like a tank and has a good solid feel to it with an extremely large display and is extremely cheap best of all she can drop it all day long without any worries. Let me know if you can't find one in your parts I'll send you one all you'll need is a sim card.