Transition lenses in eyeglasses have been around for many years now. The mechanics behind transition lenses is that certain chemicals in the lens interact with UV light from the sun and turn the lenses dark when you go outside and back to clear when you go inside.

This is a great accompaniment to sunglasses, as it is not always convenient to be carrying around multiple pairs of glasses with you, especially when going from inside to outside frequently. However, there are some drawbacks to transitions, including the fact that they don’t get as dark as sunglasses, have some difficulty turning dark in the car, and have a tendency to keep a slight constant tint even in dark conditions.

Vistakon, the optical wing of Johnson & Johnson, came out with the first transition contact lens a few years ago.  They work well for some people, and don't seem to do a lot for others.

Just a couple personal thoughts: It can look a little strange, depending on the person and the eye color. The material itself turns gray, and therefore the person can be walking around with eyes that look darker than normal. On the plus side, though, this would be good for people limited by high prescriptions and who have difficulty with peripheral vision and glasses, yet still want the transitioning technology. On the other hand, wearing clear contact lenses with sunglasses would provide better sun coverage with the ability to remove the sunglasses when desired.

An area where transition contacts might become very useful is in outdoor sports.  For any athlete who is playing an outdoor sport where the lighting conditions may change from day to day or even within a single game or event, these contacts might significantly improve the ability to perform. This is especially true in any sport where there may be significant contact or rapid head movement that can make it difficult to compete in sunglasses.  

While still in its infancy, transitioning contact lenses look to be a promising technology.