Unless I'm mistaken, this is a feature of the Blackra1n jailbreak....known as tethering.
Apparently, everytime you turn off your iPod, you have to re-jailbreak it again next time tou turn it on
If you are away from your PC when you turn it on (& cannot re-jailbreak) I guess it can't be used...
If I am right about this.....is it possible to have 2 'profiles' for your iPod?
One that is jailbroken & one that is not?
That way, if you are going to be away from your PC you could sync to the un-jailbroken 'profile'......& still use the iPod
Hope someone can clarify this!