I would expect the standard scrolling gesture to scroll the iframe -- unless the iframe is already scrolled all the way in that direction, in which case it applies the scroll to the iframe's container.
This behavior is intuitive, if slightly inconvenient in some cases, which is exactly the trade-off I'd have expected Apple to make. Requiring an unfamiliar multitouch gesture for a common operation strikes me as the kind of "let's do something cool because we can" geekery that got us middle-click-to-paste in X Windows.
I'm curious: what does the iPhone use to scroll an iframe in an iframe? Three fingers?
