That's the big question, isn't it.
This is what they say, but Sprint's coverage map alone doesn't help with this question.
from the website:
How much does domestic roaming cost? International?
Domestic roaming is free. Although we don’t offer cellular service internationally at this time, your republic wireless phone can place calls over Wi-Fi from anywhere in the world to the U.S.
Its pretty clear this is still way early in their product offering and they're waiting to see what the response is and then hash out these details. I'd love the WiFi option, but I'd need to know once I walk out the front door that it will work. Do you loose a connection when WiFi drops, or the signal transfers over to cellular (and reverse). Will they let you sign up if you're outside sprint coverage, would you want to, and so on.