Here’s how to revert to a previous firmware version on your iPod Touch (iPhone?), even when apple states in it’s release notes for the new (beta) firmware that it’s not possible to revert 
The trick is to correctly enter your iPod into the DFU (Device Firmware Update) mode.
- start iTunes
- connect your iPod to your computer
- press the power button and home button down for at 10 seconds
- release the power button and?keep the home button pressed until iTunes shows a message that it has found an iPod in recovery mode
- option – click on the restore button in iTunes
- select your firmware file (in my case 2.2.1), if you can’t find it, download it here and rename the .zip file into .ipsw, then select it in iTunes
- the firmware update starts, but keep patient! (sometimes it seems nothing happens, but wait, you will see it does)