They do screen repairs pretty quickly and easily. Saw a guy do one in minutes recently so don't expect a new phone for that issue. If BestBuy can repair them, Sprint should be able to with no problem. The issue with repairs is always whether they have spare screen parts on hand.
If you ask politely and catch them at the right time, they just might give you a replacement. It's not going to happen anywhere where there's high demand for the Evo, but I just got one replaced in Langhorne, PA Corporate store because they couldn't fix my (broken USB port) and didn't want me to have to wait for a delivery. They didn't have refurbs on hand at the time. I'd suggest you call and find out if they have refurbs in the repair center. If the answer is no, get over there asap and ask if they can give you a new one if this one can't be repaired.