It looks like Apple will give iPad owners the next major software upgrade that comes out after you purchase. So those of us that are buying right away will not have to pay to upgrade to 4.0 but we may have to pay for 5.0.
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.