Take a look at this little prototype robot that can move on the land by rolling around and then transform into a helicopter. Courtesy of researchers from the Center for Distributed Robotics at the University of Minnesota.
(Rolling Robot transforms into a helicopter video)
An ideal platform would spend most of its time on the ground but still be able to fly when it needs to...
As it turns out, it was actually more efficient to design the robot with two completely independent motor systems than to try to design a transmission that would allow the low speed wheel motors to power the rotors or vice versa. And even then, it's still extremely complicated: the rotor folding mechanism cost almost US $20,000 to create.
Certain phases of this little robot really remind me of the surveillance robots in Philip K. Dick's 1960 novel Vulcan's Hammer:
On the chest of drawers something was perched. Something that gleamed, shiny metal, gleamed and clicked as it turned toward her. She saw into two glassy mechanical lenses, something with a tubelike body, the size of a child's bat, shot upward and swept toward her.
(Read more about Dick's robot tracking device)