Apple have begun to distribute the iPhone OS 3.2 SDK, complete with final (aka "Golden Master") support for the iPad, as well as a number of new iPad-related files on their developer site. The new offerings include a firmware build for the iPad - build 7B367 - and the End User Licence Agreement for Apple's software. Buried into this latter file are apparent details as to how Apple will be managing future iPad software updates, and the good news is that, rather than an iPod touch-like paid model, it seems the tablet should (at least initially) get free firmware boosts.
"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." Apple iPad EULA
The text has drawn comparisons with Apple's iPhone policy, which so far has given users of the smartphone all of the major updates to the iPhone OS at no extra cost. This contrasts with the iPod touch, owners of which get minor updates free but have previously been asked to pay for more significant updates.
It seems that - for 3.x and 4.x software at least - the iPad will follow the pattern of the iPhone. However when - eventually - the iPhone OS hits version 5.x, owners may be asked to pay for that; it's not definite, but Apple certainly leave themselves open to such an arrangement in the terms of their agreement.