You should definitely be able to go back no problem. the MR2 is a manual flashing of the firmware to the 1.5 software (ROM) (officially released from Verizon) to push the software onto your plugged in phone. You can do this with any computer technology (your PC's bios firmware, your CD/DVD firmware, all kinds of phones, etc)
The OTA may or may not work, if it doesn't just use the MR2 ROM update to go back to 1.5, and the OTA should work just fine.
.
Here is why I am not buying this theory until someone actually goes through the 2.1 --> MR2 process and shows it works:
What if it is the case that the MR2 updater is NOT the full 1.5 ROM, but just a patcher of the 1.5 ROM. If so, it would need an existing installed 1.5 on your phone to work, and it couldn't convert 2.1 to 1.5.
Right?
And in fact, the OTA MR2 update came through quite quickly, so it seems unlikely to have been the full 1.5 ROM.
Now, maybe the non-OTA updater that is avail on the pcdphones website really is much bigger and has the whole 1.5 ROM in it? I don't know.
But that's what holds me back.
Slugdoc
(not a huge fan of irreversible steps.)
EDIT ** The file on pcdphones is 119 MB. Makes sense as the whole ROM.
So, um, never mind, should work fine.

Will update to 2.1 when I get home.
RE-EDIT ***
Too hasty. Apparently may not work per Caddy and another new thread. Show me the proof first.