As long as you're sure it's the phone and not your wifi router/access point, it should be pretty easy to show someone at the Sprint store that it's defective, and I would think they would give you a new one. Just bring it in to a store.
Can it connect to other wireless access points? Also, can other devices connect properly to your home WAP? These are things that the Sprint folks are going to want to know.
I had the same issue took it to Sprint and they tested it out and had the same problem they replaced it for free and gave me a small credit on my account
I think that will be a hard sell....they will probably try & connect you to the store's WiFi to see if it is capable....if you can connect, then they may say it's a problem with your network/equipment.