So I'm thinking about rooting my Evo. Is this a completely legitimate thing to do? Just seems like it surely must be going against some kind of ToS or some such.
In the US it's always legal to do:
http://www.intomobile.com/2010/07/26/iphone-jailbreak-and-android-root-declared-legal-in-us-feds-exempt-smartphone-hacks-from-dmca/
As far as handset warranty, rooting voids it, but you can always restore to stock unrooted if something isn't working right and you need to take it in for warranty service. If it completely breaks, they'll likely just replace it, and wont be able to see if it was rooted or not.
If you do root, definitely think about custom ROMs.