I've been searching for you and here is what I've found:
iPod firmware 1.1.3
A little forum talk about "how to downgrade" here
And a comment from a user:
You should be able to restore back to the 1.0 firmware using the ipod updater, when you run it just use the option to RESTORE IPOD to factory settings. It will format your nano so you will lose all the music on there
The only thing I don't know is if the latest 2nd Generation Nanos will run the earlier 1G firmwares?
If you nano defiinately only has the 1.1 firmware on it (1.2 if the latest) do not use itunes to update as I don't think you will be able to rollback.
I'm not a nano expert but I would recommend you have a good read of ipodwizard.net as the forum there is where you will find all the answers you need.
Messing with your firmware could easily result in a dead ipod !!
Hope this will help you!