This only reverts your SPL/HBOOT back to 0.33.0012 after you've installed the Korean FRF91 files and ended up stuck with 0.35.0017 SPL.
Thanks to the guys in the Desire forums and help from the Nexus Q&A, I have reverted my phone to the original HBOOT so I can apply custom roms again.
First you need to root the Korean FRF91 that's on your phone using Paul's instructions for superboot FRF83.
If you are rooted already, skip this step.
You need to download the EPF30 update, save it to your SD card as PASSIMG.zip.
Create a new mtd0.img file through this site
I used the values, CID: 11111111 and Rom version: 0.22.997.0
Save the mtd0.img file to you PC.
Download the flash_image binary (right click, save as)
Make sure it's saved as flash_image, and not flash_image.txt
If it has a Notepad icon, then it's not saved right.
With mtd0.img and flash_image in the same folder, type these commands from within that directory:
adb push flash_image /data/flash_image adb push mtd0.img /data/mtd0.img adb shell chmod 755 /data/flash_image /data/flash_image misc /data/mtd0.img
No worries, this goes away after reflashing the EPF30 Nexus One PASSIMG.zip package.
Power off your phone, then turn it on by holding VolDown and pressing Power.
It will start up in HBOOT mode and will prompt you to flash the EPF30 PASSIMG.zip update.
Once this is finished you can root the EPF30 system through Paul's superboot instructions above, then use Rom Manager from the market to apply a custom recovery again.
Now, remember not to flash the new HBOOT again.
I'd recommend flashing the 5.08 radio for the N1 through other means. Search the forum to find the instructions.