Lukas Mathis touches on this visual scrollability hint in his discussion of potential solutions to usability issues with Lion's new hidden scrollbars:

http://ignorethecode.net/blog/2011/08/01/invisible_scrollbar...

Showing a peak of extra content to indicate that there's something you're not seeing may not be deliberate, but it's definitely desirable when you've got no other permanent indicators. That said, given Apple's usual attention to detail, I wouldn't be surprised to find that this is a deliberate design choice.