Supposedly, Sprint can't stop people from tethering while on the 4G network due to some activation issues that they don't have 100% straight yet. However, they are saying that without paying the fee that they can stop 3G tethering. Some people here have stated that they're able to tether regardless of which network they're on regardless of paying the fee, YMMV. However, regardless of what works and what doesn't right now, I'm fairly certain that Sprint will get it straightened out before too long and those using it without paying will find it inoperable at some point in the future.
For what it's worth, I'm paying the fee and I'm tethering my iPad right now to post this message. If everything works out like I think it will, I'll be canceling my home Internet and just using the phone's hotspot feature.