Or sign up for a new google voice account, and port your Sprint number over to it (if it's important to not have to tell your senders the new number). Then make sure the new unlocked phone is an Android phone. It should come with a gtalk app already installed, which will receive your SMS messages.

Alternatively, I know a Sprint user who went to Europe, and rented a world phone before leaving. Sprint charged him $90 for the world phone for a month, so that he could continue placing calls and receiving calls as if he were in the states. Presumably SMS would also work under that scheme.