I doubt it would brick the phone, haven't heard about that happening. I have heard that happen to Hacked Wii's though. The root exploit blocks the OTA updates... if it doesn't, theres a simple 1 line of code you could do to block them (basically, move the file so that the system doesn't find it when it gets pushed out).
Here is my recommendation:
root as soon as you feel comfortable. Maybe don't do it in the store (even though technically you can

) check here to make sure it was successful for release day Evo's first. Then, do it. Make sure you do it before the OTA is pushed out though cause it
could potentially block the exploit and a new one would be needed. However... I don't think it will, and here's why. Practically the same exploit for the Evo was found to work on the Hero for 2.1 root (TODAY!!!). It seems like its a general 2.1 exploit, not one specific to the Evo. Potentially, it may be the same or similar exploit found for the Incredible and it could potentially be modified like it was for the Hero to work on the Eris (those poor souls). Its just a theory, but it could be an exploit bigger than the 3hr time-to-hack was.
Unfortunately:
If you root, you technically void warrantee
You can unroot via flashing an RUU (which is windows only) and none is publicly available to my knowledge.
Should you have a major issue with your brand new rooted Evo... you may not be able to do anything
Fortunately, since there aren't very many appearance changes with just rooting the phone, you could probably remove SU Permissions and no one would be the wiser on Sprints end.
So there you go. Let me know if you need any help or have any questions. I may be scarce through Wednesday since I need to take my Pharmacy Boards so I can get a J-O-B and study my arse off, but if I browse on here for a mental break, I'll see what I can do. If you couldn't have guessed... I'm not a programmer, just someone who gets most of this stuff
