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.
It's a little muddy, but the take-away is that Apple will provide free upgrades of the iPad's system software (like it does for the iPhone) only up to the next major revision in software. Anything beyond that, and users will have to pay to get the latest features. Apple already employs this model with the iPod Touch. I haven't seen a single report on the percentage of iPod Touch owners who have chosen to pay Apple $10 to get the software updates. Apple hasn't shared that information that I am aware of.
The iPhone, on the other hand, offers system updates for free and has since the beginning. This is the model used by just about every smartphone maker out there. Google, Palm, Microsoft and RIM let users update their base operating systems at no cost.
Will charging for upgrades work with the iPad? Well, it does for Apple's laptops and desktops. Apple users appear to be all-too-happy to pay for system updates. Can that OS sales success translate to the iPad? Only time will tell.
I think it's safe to make a few assumptions here. The iPad is shipping with iPhone OS 3.2. iPhone OS 4.0 is expected to debut later this year, likely when Apple announces a new iPhone in June or July. Apple is saying that it will allow iPad owners to update to that system, because it will be available in just a few months. Beyond that, however, users will have to cough up some dough.