Are you a developer trying to install 4.0 firmware on your 3rd gen iPod touch? You may run into install problems based on device eligibility. That means that Xcode or iTunes will complain that it cannot install the beta firmware onto your device and may tell you that you are attempting an ineligible install.
Whether using Xcode or iTunes, your iPod will be left in recovery mode -- and you might think you'll need to restore the device and wait for however many hours you'll need to re-sync all your music, videos and apps back ...
Good news: you can kick yourself out of recovery mode and back to your previous firmware install without doing that. Just use a tool like iRecovery. As I discussed in an older post on TUAW, you can use iRecovery to set your iPod to boot back into normal user mode.
Download iRecovery from iHackintosh. Run iRecovery from the command line and supply the -s flag. Wait for the "]" prompt, and do not type anything except a carriage return (press return, wait and repeat if nothing happens within 30 seconds) until you see that prompt appear. It helps to only have one iDevice attached to your OS X desktop.
Once you have the prompt, enter the following commands:
] setenv auto-boot true
After exiting iRecovery, you return to the command line. Reboot your phone by pressing the home and sleep buttons for 10 seconds, per the instructions on the iHackintosh, and your iPhone should boot back to its previous firmwall install without re-entering recovery mode.