Can Apple update the firmware on a Made for iPod device using the iPhone?
The accessory has to meet very strict design requirements from Apple to connect on the 30Pin connector. And the approval requires to work with the latest version of iOS. Due to this, the earlier versions of iOS may give you some error messages in some conditions.

Not having seen your accessory, I would take a guess it would be due to the resistors that are connected to the USB D+/D- lines if it is only a charge only device (or) the authentication chip would be the latest version 2C