I suspect once you root your EVO and download a hotspot app, you will be good to go and bypass the $30/mo charge.
The reason I say this is, because, not many phone buyers actually look into the phone so far as to root it. Most users don't even know what rooting is, so for Sprint to use resources (and money) to find the people who do root to get free hotspot on their phone will more than likely cost Sprint just as much money as it would if those people were paying monthly for it.
.. This is my opinion, and once a few people do successfully root their EVO 4G's and are able to be a mobile hotspot without paying, I will follow right behind them and do the same.