Is there such thing as an automatic transmission for bikes? Has anyone ever designed or manufactured an automatic transmission (or automatic shifter?) for cycling?

I imagine such a device would automatically change the gearing to allow the rider to maintain constant cadence or constant torque on the cranks.

I used to daydream about the existence of such a system many years ago when riding my first (battered, second hand) racer as it was such an ordeal to change gears.
http://www.landriderbikes.com They were very heavily advertised several years ago but currently they seem to show up more on craigslist than on TV.