you will most likely gat a signal at any and every island you go to but be prepared to pay a kings ransom for international roaming. i brought my phone in case of emergency because of a sick father and it rang with messages every day we were in ports.
Call your carrier. Mine, Cingular, had a temporary package I could add that included most of the Carribbean and I think one for some of Mexico as well. My phone seemed to work in all ports. . .(although I didn't know the syntax for international dialing and did need it) and with the package it didn't cost much. I think it was roughly $10.00 and was adequate minutes, though I don't remember details.