I did this just out of reaction...The option to install came up and i hit it not thinking, then my phone reset...ARGG..
At the time, i had the AmonRA's latest Recovery installed (from when i installed a VR_Kindom Rom)... so being it started, i patiently waited, watched, and held my breath.
Turns out, it broke my recovery, un-rooted my phone, but was a fully functional ROM still using 2.3.3 when done. I shut it down to verify the recovery, and it just gave me the famous red exclamation with the triangle.
Luckily I've been doing this for a while, and rounded up a stock rom for 2.1, named it PC36IMG.zip, let it fully do it's thing through the new hboot, and after it finished, i reran the old unrevoked3, which gave me back my root access, readded a recovery (clockworkmod), then i fully flashed a nandroid backup i had just before the update, and was back running and flashing custom roms again...
Hope this helps !!