Depends.
If you pay full price for new phones then they will unlock within 48 hours - or sooner if you call them directly.
If you are on a lease, or Easy Pay or a contract then the phone is indeed locked. However, as long as it's not an iPhone 4, 4s or 5, you can have the phone unlocked by Sprint when it's paid off. It will be done automatically.
This is a new policy that just started in February.
The Sprint iPhone 4 does not have a SIM so cannot be unlocked. The 4s and 5 can achieve a full unlock with a workaround. They have to be inactive and you call Sprint and request an inactive international unlock. These have been converting to full unlocks.
In the case of the iPhone 4s, this however will forevermore render it useless on Sprint's network.