Apple has released the shipping version of it’s OS for the iPad. The licencing information notes that, updates to the current and next version of the OS will be free, but they may charge for newer OS versions. iPhone users are currently not charged for OS major upgrades, but iPod Touch users are. It seems that iPad owners may have the same faith as iPod Touch users.
In the piece quoted below it would seem to indicate that for example if iPad shipped with version 3.x that apple will provide free updates up to and including version 4.x, however version 5.x may have a charge and also charge for versions beyond version 5.
Apple will provide you any iPad OS software updates that it may release from time to time, up to and including the next major iPad OS software release following the version of iPad OS software that originally shipped from Apple on your iPad, for free. For example, if your iPad originally shipped with iPad 3.x software, Apple would provide you with any iPad OS software updates it might release up to and including the iPad 4.x software release. Such updates and releases may not necessarily include all of the new software features that Apple releases for newer iPad models.