Planetary gearing has been used in bicycles for over a century. A planetary mechanism built inside the rear freehub provides clean and reliable gearing, albeit at a slight efficiency loss versus derailleur gearing. These gearhubs are extremely popular in many parts of Europe, where utility cycling is the norm.
The greatest example of hub gearing is the Rohloff Speedhub, which features fourteen ratios with an overall range of 526%. It is a true mechanical marvel, and several examples have done over 100,000km without major servicing.
Automobile automatic transmissions (with the exception of Honda transmissions) have also used planetary gearing since their introduction in the 1930s. The Prius is innovative because of its design utilizing MG2 to alter the output speed continuously, not because of its use of planetary gears.
At any rate the Rohloff hub is really cool, and I'm glad the technology has finally trickled down into popular, attainably priced planetary gear hubs like the Shimano Nexus as well.
The greatest example of hub gearing is the Rohloff Speedhub, which features fourteen ratios with an overall range of 526%. It is a true mechanical marvel, and several examples have done over 100,000km without major servicing.
http://en.wikipedia.org/wiki/Rohloff_Speedhub