1) Are you absolutely sure you want accept the OTA? You have to understand that if you do, it's not only going to update your SW version and radios. It's going to update your hboot version. If you have any nandroid backups, they will become useless, and you will not be able to flash them. There *is* a way to get them back, but you really have to know what you're doing, and the situation can be avoided by not accepting the OTA. If you'd like to see the procedure for updating the radios and software version manually, see the Rooting for Dummies guide, section 6 under after rooting. As you've apparently already flashed the S-ON tool, you'll have to flash unrevoked-forever to get S-OFF before you do it.
2) If you are that set on unrooting, you have two options.
a) Flash the correct PC36IMG.zip, found here:
MEGAUPLOAD - The leading online storage and file delivery service
The reason the one you're trying to flash isn't working is because it's a little older. Based on the date, I'd guess it's hboot 0.93 (I'd have to download the whole thing to verify). You can never flash an older hboot this way.
b) Flash the stock recovery for hboot 0.97 (sw version 3.29), found here:
http://androidforums.com/evo-4g-all-things-root/259446-clockwork-amon-ra-pc36img-files.html
The first option will wipe everything, as I'm sure you know now. The second option will not wipe anything. It will just give you the stock recovery, allowing you to accept the OTA update. You theoretically could even leave S-OFF, if you want. Thing is, you'll probably want to rerun unrevoked after the update, anyway, to get things back to the way they should.
NOTE: I highly recommend that you do option 1. Updating manually will not change your hboot version. If you accept the OTA updates, you will be updating to a hboot version that was not meant to be downgraded. As I said, it's technically possible to do so, but it's not as simple as updating manually would have been in the first place. Any nandroid backups will no longer work.
