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I have been adding Light Emitting Diodes lighting to my house over the last year.  I have been using the OptiLED James speaks of.  They are not very bright, but work well for low level lighting so a person can avoid turning on other lights.  My current project is using motion sensors to gently light the walls at night when a person enters a darken room.  Unfortunately I find this work more of an exercise enjoyed by us electrical engineers or electronic hobbists.  The products are not ready for the masses, but they have great potential.  For example, with LED you can have not only a dimmer but select the color tone of the light as well.  Using DMX lighting control can give a person great flexibility in lighting.

Cool things about LED are energy efficiency, small size (true point source), long life, and ability to be intergrated into fixtures.  By building in spares into a fixture a lighting appliance (aka lamp) could last a million hours.

I've tried CFLs over the past 5 years or so and have two problems with them---the bases are often too big to fit most of my current fixtures, and they're not good for use outside because of cold temperatures affecting their life. They hardly seem to last any longer than regular incandescents outdoors.
Do the LED bulbs last outdoors?

LED work very well outdoors.  These devices have been doing their magic in Vegas for ten years.  Those at the Young Sign and Daktronics have been pressing envelope making LED signs more and more like TVs in Vegas and sporting events.

LED can supplant CFL.  It may have a larger impact than CFL as LED may replace fluorescent technology which has been used in commericial and government buildings since the 50s and 60s.

LED do very well in cold weather.  They also pose little fire danger.  They are showing up in gas stations, high end retail, restaurants, bars, signal lights, and cars.

Costs are high, but so are the savings, as well as the life.