Spoke to a manager at my local Sprint Store today about this
The SIM is unlocked. No problem at all to purchase a local SIM card for use when travelling. However, when using a SIM card other than the one that comes with the phone, the phone will not be able to calls to your regular number, only the number connected to the SIM.
You can also get an international plan using the SIM that comes with the phone (with your own number), and even upgrade and downgrade to/from the international plan to cover the time period of your trip. He did say that would likely be more expensive than the rates with a purchased SIM card.
He mentioned that some international phone companies (like Vodaphone) would be able to provide you with a SIM card that will cover multiple regions, so you won't have to have a buncha different SIMs for each country you travel to.
All data usage on a purchased SIM card will use local data rates. Data usage on the SIM card that comes with the phone will be at international data roaming rates or the rate of the international plan you have.