My colleague pointed me to this search ...
http://www.google.com/search?&ie=UTF-8&oe=UTF-8&q=%22VNC+Desktop%22+inurl%3A5800

... which finds VNC desktops. Does anyone know how to make good use of this "Google hack"?

Not unless you know the password to get onto the machine.

(Or would gaining access to those machine be a "bad use" of it?)

Although, some dumb users use the same name of the machine as the password. Take this for example:

VNC desktop [awos]
12.219.148.180:5800