If you want to go back to 20110909, you have to update two SPI, one is for going back to android2.2, one is for working on android2.2, attached update_spi_all_2.2.img , don't unzip and change the name to update.img
Used this then removed sd card when rebooting after update. Then put old firmware as update.zip on sdcard and restarted and it updated back to old Android. Worked ok.