Just got back from the Dominican Republic. Leave your phone home unless you own Verizon Wireless!
We had a data plan but the afternoon we arrived, I got a notice that one of the phones (a teenager) had gone over $150. In six hours! And that's just data! I kept everybody on Skype from our computers as much as I could ($2.00 per minute -- no plan available) and tried to control texting (.50 per SMS -- no plan available). Wait, it gets better -- Verizon cannot tell me within a thousand dollars how much our 5 day adventure cost us -- the bills will keep trickling in from the Dominican Republic over the next two months.
My advice again is to not take or use your phone (except in WiFi mode) there until Verizon can get a better deal from the Telco in DR. Yeah, you can get Skype and a number of other services that are voice and can call out at any time, but it does take a bit of geekiness to get a number that you can have folks use that will get through to your phone over WiFi.