The movies Innerspace and Fantastic Voyage both featured miniature submarines, piloted by miniaturized humans, roaming around inside a live patient. As the trend continues towards unmanned autonomous vehicles (UAVs) both in the air and in the water it seems the movies had half the story right – that of the miniature vehicle. For example, researchers in Japan have developed a self-propelled endoscope [source: Popular Science] that can swim mermaid-like through our digestive system.
The other half of the story – miniaturized human pilots – may make for a more exciting storyline but it looks destined to remain fiction for quite a while longer – along with the elusive Star Trek teleportation system. Full-sized humans are becoming increasingly scarce in airplanes thanks to advances in unmanned aerial vehicles, so I can't see how miniaturized humans are going to have a role in miniature UAVs anyway.