From what I have heard Apple did not allow access through USB for any syncing other than with iTunes for example SPB Wallet/ SBSH SafeWallet both use your Wifi network via Bonjour.
One of my forum buddies at SPB found this
Link which advises that with the release of 3.0, some apps have now been enabled for USB Sync, he advised the following 3rd party apps supported syncing via USB.
* Good reader
* FileAid
* iStoreage
He believed they are using the external Accessory API, so this may mean that some 3rd party devloper may make an app that could be used to open the USB sync for internet...
I have tested to see if my iPhone 3G would be able to use the interent via USB with OS 3.1.2 & both Wifi & 3G turned off... the answer is NO.
I doubt this will be added as most people would have access to either their 3G data plans (I currently have 2GB plan) or Wifi network. I could be proven wrong in the future (& I hope I am) that Apple may release this option.
Cheers
Jace