SPrecovery and clockworkmod are ways to backup the system, restore the system and install custom roms and frameworks. If you are rooted you will have one of the two and just power on the phone holding X boots into SP or clockwork.
As of right now I believe there is no way to root the official ota 2.1 update .... so if you updated there is no way to undo it now. There is also no way to revert back to 2.0.1 from the official 2.1. If you are still rooted and never did the official update then there are roms out there with ESE81 included now so you can install those if you want. You do have to reactivate when you boot up again .... but it calls *228 you just have to press 1.