Too late now, but maybe this will help someone in the future... I did the exact same thing to my phone night before last. First I rooted in 2.1 with the 1 click. That was so easy that I thought to myself, what the hell and decided to go for the 2.2. leak upgrade... went perfect till reboot, at which point I got an "M"...
I then went for the a complete reverse back to 2.1 with the motorola boot loader... at some point I got the File Corrupt, Battery Low Cannot program and she was DONE! - Complete Brick...
I thought about the low battery msg on screen and it brought me to here on a google search. Bummed out, I kept searching with some different search terms and noticed a Motorola page where someone was experiencing a similar issue with the battery low msg.
Long story short >>> if you are bricked with the low battery message, go get another battery, recharge your in ANOTHER phone because if the firmware is not running on your phone, then your phone will not charge when plugged in to anything..
OR, you can do what I did >>> Cut the small end OFF of a USB cable, CAREFULLY shave off a little coating on the Black (-) and Red (+) wires and wrap them around the end of the - and + post on your battery... Use a piece of tape to hold the wires steady on the top of the battery and CAREFULLY insert it back into your phone so as to keep the wires from moving around and grounding out.
Plug the other end (the regular USB end) into whatever, wall charger, PC, etc... wait for a minute or two and reboot the phone. You can now plug in the other USB tether cable into the phone and PC and proceed to Boot Loader on the phone... should not be any error msg and should also read Battery OK, ???program.
SIDE NOTE... The OTA 2.2 update was released by VZW 4 minutes after I started the ROOT/2.2 upgrades. I was unaware and wasted a whole day unbricking my phone. All ends well though, now running OTA 2.2 dancedroid
but I cant get root now because of some driver issue that seems to be blocking the Debugging/PC Mode to allow 1 click Root to run... it just sits there trying to install a driver that either is not there or is not recognized.
Any Suggestion for Rooting and OTA 2.2 Droid X ??? Despite my crash course in technical language over the past 24 hours, I am no techy.