Sense you already have a 800 number i'm guessing that is the number your customers expect to call. If you do change it to a cell phone it could be some time before your customers catch up with the new number.
As far as everbody or most everbody has a cell phone i wouldn't bank on that, just their are a world of folks still on dial up internet. My guess is theirs a world of folks that either don't have a cell phone or a prepaid one like me. Thing is i can talk all day for a 2.00 a day charge, but do i want to spend 2.00 to call you and it's the only call i make that day ? If their like me they have a cell phone prepaid or other wise is for one reason only and that's in case they have a emergency and the rest of the time it's turned off.
Another bad thing about a cell phone is i giving your credit card number on one, if it bleeds over on to some body else's phone and they do just like a cordless phone that person can get your CC number. Bottom line is changeing to a cell phone right for your business or not only you know that. Plus i'm sure you can take the 800 number charges off of your taxes if you don't already, but check with your tax people to make sure.