I seem to remember my computer (Running Ubuntu 10.04 with NVidia drivers, and the standard DE) doing that at one point. Through changing my external monitor, the resolution used was larger than my monitors, and I could scroll the viewport by mousing over the edge.

I never bothered to figure out exactly how enable/configure this feature intentionally, because I saw it as more annoying and simply reset the resolution.

I also remember being on a mac, wherer I could zoom in the entire screen, then scroll the viewport.