I find that in many applications where I scan scroll on my iPhone, there is a subtle but effective way to telling me that visible: the last line in the scrollable area is only half visible. That quite effectively communicates that there is more to see, and once you start scrolling, you'll notice that there is an indication where you are in the list.
I know that this does not apply for all scrollable contents. For instance, on a webpage, you cannot control if a line on the bottom will end up in the right position to be only half-visible. However, I get the distinct impression (just playing around with my phone to see what happens now) that here another technique is used: the scrollbars that are visible during scrolling, are also briefly visible when the content is first rendered in the scrollable area. I guess that also serves as a reminder that the content is scrollable. At least Safari uses this technique.