Its true. Lego really do have a Solar Panel.
I am not sure of the full specifications - only that it is part of their Dacta range - the eLab sets specifically.

It is part number 32339. It is in green, and is available in the set 9912-1.

According to the Lego Site:

The solar cell consists of seven (7) solar cells and a diode. They are connected in serial so the total volt = 7 x 0.4 V (a total of approximately 2.8 V).

• 3 V, 200 mA at full sun outdoors
• 3 V, 100 mA at full sun indoors through window
• 2.5 V, 8 mA with light from 60 W incandescent lamp with 25 cm distance to solar cell (2000 lx)
• 2.5 V, 40 mA with light from 60 W incandescent lamp with 8 cm distance to solar cell (10,000 lx)

Lego also offer the following cautions for its use:

• The solar cell is not waterproof
• Do not bend excessively or push hard on the clear plastic window
• Do not step on or otherwise place heavy weight on the solar cell
• Ordinary incandescent lamps produce a lot of heat and should only be used for short periods and at a distance of at least 8 cm

It can be purchased by individuals at Bricklink or Commotion Group. Schools, Organisations and Colleges should contact their Dacta distributor.

Although the standard Lego Motor (with 5225, RCX etc) runs at 9v, it can be run from these lower voltages. Combine this set with the Lego Capacitor and a few other components - and you can build a Lego BEAM Robots example.

Click to Enlarge

Thanks to Peeron for set and part information.