As soon as it gets rooted you can tether. Whether or not it will use the actual Sprint hotspot app or not I couldn't tell you. I am not sure anyone knows how long it will take to root the phone. It is usually not too long on a new device. A lot of times these things are rooted before they even hit the market but who knows?
Now, whether or not HTC allows us to unlock the bootloader quickly or whether they leave it unencrypted and a wonderful dev can get us a flashable recovery is the other question. I would guess with that a rooted ROM could be flashed that day but I am not entirely sure, nor is anyone else.