My cell phone is a corporate Blackberry, so I never actually see the bill (although, I was told that when 6 of us travelled to India in March for a week, our combined charges came to $4,500). I try to act responsibly with it, so I generally leave it switched off all day, and then switch it on for 5-10 minutes in the morning while in port, allow the email to synchronize, then I switch off the wireless and read and respond to my emails. Another 5-10 minutes to allow the email to sync again (so my responses get sent) and then I switch it back off again. If I need to make any calls, I do it during that morning period.
If I do need to make a personal call, well, that compensates for my checking work emails during vacation.
I will generally carry it with me (powered off) during excursions, just in case I run into trouble and need it for emergencies or otherwise.