vawterf: call your cell phone carrier and ask. Cell service in the Caribbean is sketchy at best. Some islands have cell service, some do not. Be very careful about roaming charges! Using pre-paid calling cards, or using the calling centers on the islands are usually better choices.
taz - i can't offer any info on cell phone purchases in the Caribbean, have never done that. Don't go to the Caribbean thinking that you will find a Best Buy or Circuit City or any type of store like that, you will find small chain & specialty stores with limited stock. St. Thomas has a KMart. The key is knowing your prices before you leave home, sometimes you can get a better deal at the big chain stores back home.