A user would still have to understand why trying to grab+drag to scroll would work on one part of a page but inexplicably not on another part (eg, a Flash ad).

A user would still have to understand why trying to grab+drag to scroll would work on one part of a page but inexplicably not on another part (eg, a Flash ad). Most likely it would be a crap shoot between pages that took the iPhone/iPad into account, and those that would confusingly not work.