For the iPhone, their price has been considered-included, as an accounting matter, in the ongoing revenue Apple shares from the mobile operators. But, for the iPod Touch, upgrade fees have applied to get new features -- with the stated rationale being accounting requirements.
The iPad(Wifi only) is clearly more like an iPod Touch. But even the iPad(Wifi+3G), with no long-term contracts and buy-as-you-need-it service, may not have any ongoing Apple revenue share.
All iPad owners may be on the hook for a fee when iPhoneOS4.0 arrives.