I read on XDA that it might be possible to downgrade from 2.1 to 1.5 using a file that's extracted while running MR1 RUU (not MR2).
I have NOT tried this, but you're welcome to be the guinea pig.
1. With your phone not connected to your computer, run the MR1 RUU (not MR2). Get to the point where it says it can't find your phone and wants to exit. Don't exit.
2. In Windows explorer, go to:
c:\Documents and Settings\yourusername\Local Settings\Temp
3. Sort by date, and the most recent subdirectory should be from no more than a few minutes earlier. It'll be something like "{C4AEBEB0-34E4-482A-811F-C426B9FA87AC}". Open it and find "rom.zip".
4. Move "rom.zip" out of that directory onto your desktop or something and then exit the MR1 RUU.
5. Rename "rom.zip" to "PB00IMG.zip", put it on your SD card, and follow the directions that we all used to upgrade in the first place.