It depends on how compelling the content is; if a user is really engrossed, they'll be happy to scroll down pages of content. If they're not, keeping everything

It depends on how compelling the content is; if a user is really engrossed, they'll be happy to scroll down pages of content. If they're not, keeping everything above the fold won't be enough. Naturally there are limits to how much a person will scroll no matter how compelling the content. With 37signals, they probably feel their copy is engaging enough and the design offers enough varying contrast as a person scrolls down that having someone scroll that far isn't an issue.