The scrollbars aren't the most obvious cue you can have ;-)
The scrollbars are only one of many cues. I've observed lots of usability tests where there are scrollbars and people don't scroll because there is a natural break in the content. When the reader is unaware of the nature of the content they finish when they believe the content ends.
If you look at the 37signals page in contrast http://cl.ly/2W0w2M0w0m3z0d1D1G3r the entire bottom part of the screen is obvious "broken" by the bottom screen edge. Both the page ending in the middle of the text and the grey boxes that surround each option that are not completed are cues that there is more content. To even a very casual reader it's obvious that there stuff below that you can scroll to see - with or without scrollbars.
The scrollbars are only one of many cues. I've observed lots of usability tests where there are scrollbars and people don't scroll because there is a natural break in the content. When the reader is unaware of the nature of the content they finish when they believe the content ends.
If you look at the 37signals page in contrast http://cl.ly/2W0w2M0w0m3z0d1D1G3r the entire bottom part of the screen is obvious "broken" by the bottom screen edge. Both the page ending in the middle of the text and the grey boxes that surround each option that are not completed are cues that there is more content. To even a very casual reader it's obvious that there stuff below that you can scroll to see - with or without scrollbars.