What plan are you on exactly?
With current ACII/Nationwide Plans, Sprint is Paid Roam in Puerto Rico. You will see a solid roam indicator there, indicating a per-minute charge.
Current ACII/Nationwide PRL's do, however, prefer and allow free roam on Puerto Rico Telephone Company. They cover most of the island, and like I said it's free. It should be rare for you to land on Sprint.
Verizon has a very solid native network in HI, and AK is free roaming on Alaska Digitel and ACS Wireless.
-AW