As you've noticed, recent versions of OS X only display generic folder icons in the sidebar. So why does Dropbox get special treatment? The short answer is that Dropbox uses undocumented API to accomplish this. In non-technical speak, it's a special hack that's installed by the Dropbox application. Some curious folks on StackOverflow found the specific mechanism used by Dropbox: good ol' mach_inject. The same bundle provides both the toolbar item and the sidebar icon.
