We all owe a big thank you to SpsychoS over at alldroid, who's successfully compiled a custom recovery image for the droid. With this, if you screw up your droid, you should sucessfully be able to repatch your droid with a non-signed update.zip file.
Word of warning: This recovery image allows for updating the Droid with ANY file named update.zip, so make sure that whatever you do use to recover your brick is legitimate.
Software:Images - Droid-Devs
EDIT: Since so many people are confused about how to install the custom recovery image I'm adding the instructions that are correct. Thanks to toka, tdawg and boostdscoob for posting this. I have confirmed that these do in fact work, as I've done it myself.
YOU CAN BRICK YOUR PHONE WITH THIS. (Sorry Toka)
PLEASE USE CAUTION WHEN UPLOADING THIS IMAGE
1 - place the required files (flash_image binary and recovery.img - naming shouldn't matter i kept the version number on it) on your sdcard - you can do this with a microsdcard reader, adb push, or just by connecting your *working* droid to your computer and mounting the SD card.
2 - Connect your phone to your computer in debugging mode and use ADB shell to start a root session on your phone by typing su
3 - mount -o remount,rw -t yaffs2 /dev/block/mtdblock3 /system - this mounts /system as a writable partition
4 - mkdir /system/xbin - if it already exists you will simply get an error
5 - cat /sdcard/flash_image > /system/xbin/flash_image
6 - chmod 700 /system/xbin/flash_image - validate permissions on /system/xbin/flash_image - you are the root user after all, don't need world and group executable or even read =)
7. mv /system/recovery-from-boot.p /system/recovery-from-boot.old - This will ensure that the custom image will not be overwritten on reboot.
8 - sync - flush changes to block device
9 - mount -o remount,r -t yaffs2 /dev/block/mtdblock3 /system - remount in read-only - you shouldn't need to make any more changes
10. /system/xbin/flash_image recovery /sdcard/recovery.img - will reflash your recovery partion with the custom loader.
11 - hit power button, select power off
12 - hold x, hit power button
13 - you should be greated by the custom recovery firmware