Optimus was previously rooted using sublimewolf's method: http://forum.xda-developers.com/showthread.php?t=935312
I then proceeded to brick it by using the Factory Reset found in the Privacy Screen. This forced my phone into a recovery loop.
If you can't even get to your custom recovery, your phone might be your newest paperweight But as long as you can both get to recovery and access the phone via adb, you'll be okay.

So here's how to unbrick it:

This is the flash_image: http://www.mediafire.com/?bx14hzq5i2pq3ps
And here's the stock recovery image + nand backup: http://www.mediafire.com/?ac0e2ksq0eslytk

(In my case, I was able to skip to Step 4 in the Nandroid Instructions because I had performed a Nand Backup when I rooted, therefore the necessary files were already in place)

Nandroid Instructions:
1) Unzip both downloaded archives
2) Move the BCDS-20110203-0357 dir into /sdcard/nandroid/mem=477M (also extract recovery.img and flash_image to root dir of your SD card, as you will need to later anyway)
2a) For Clockwork Recovery, place the BCDS-20110203-0357 dir into /sdcard/clockworkmod/backup
3) Mount SD into phone if it isn't already, boot into recovery if your phone isn't already there
4) Go into the "Backup/Restore" menu, and select "nand restore", confirm and wait
5) Reboot, proceed to Recovery Instructions below

Recovery Instructions:
1) Extract recovery.img and flash_image to root dir of your SD card if you haven't already done so
2) Connect to your phone via the adb shell and input the following commands:
# su
# mount -o remount,rw -t yaffs2 /dev/block/mtdblock5 /system
# cat /sdcard/flash_image > /system/bin/flash_image
# chmod 755 /system/bin/flash_image
# mount -o remount,ro -t yaffs2 /dev/block/mtdblock5
# flash_image recovery /sdcard/recovery.img
# reboot recovery

3) If everything was done correctly, your phone should boot up normally to the OS