downgrade first to 1.1.1 jailbreak run okto prep update to 1.1.2 jailbreak go into installer and run the official 1.1.3 updater.
P.s im on my itouch right now.when i get on my laptop ill write up a small tut for you
*edit*
Here I found this instead
How to Downgrade from 1.1.3 to 1.1.1 (Automatic Method)
1. Download the downgrade file from
here.
2. Extract the files to your Desktop.
3. Open the AutoDown folder and run the file called "down"
4. Press enter
5. Press enter again once the connect to iTunes shows up.
6. Open iTunes
7. Restore it to 1.1.1, shift-click and select the 1.1.1 firmware
8. Once updated choose the backup or set up as new iPod.