Could be FireWire charging--the original iPhone and earlier iPods could charge off 12V (FireWire), whereas the 3G and up only took 5V (USB). Adapters were available (although Apple didn't make them themselves)

Also with the early iPhones, if accessories were just "Made for iPod" certified but not "Made for iPhone" (which usually required TDMA emissions testing and iPhone testing) a message would pop up.