Kathy -
I used my Cingular/ATT cellphone on the island a few weeks ago - in fact I use it on every trip to SBH. When checking for coverage they need to look under Guadeloupe or perhaps French West Indies.
If you have global roaming enabled by Cingular/ATT and
if you have a GSM phone (if it's ATT/Cingular then it should be) and
if it works on the international GSM frequencies (again, it should)
then you will be able to make and receive calls, and people at home will be able to call your mobile.
Signal strength may be week to non-existant inside a villa with concrete walls. Mine rarely worked inside, but usually worked outside.
Going to the mobile phone store at the airport is worthless information unless you have an unlocked phone - and ATT/Cingular does not unlock their phones.
For incoming calls - make sure that they have both the villa number and your cell number. Make sure they know how to dial the villa number (pick up the phone, dial 011-590-590-xxxxxx).
For outgoing calls - you've got your cellphone, but you will want a backup. Go to the gas station by the airport and buy a phone card. The 15-Euro card will likely be overkill, but you'll feel better. There is a scratch-off area on the back with a pin code. I am currently using an Intercard from Dauphoin Telephone. Usage is easy - call a toll free number on the villa phone (0809-135-135), enter 2 for english, enter your 10-digit pin code, enter a phone number just like you were calling from out of state - 1-212-xxx-xxxx. Very simple, and you'll have over 200 minutes of talk time at nightime rates. I've used Pass Outremer phone cards in the past, and the procedure is similar.