The Melexis MLX90614 chip. Really it is not a cloud sensor. It is a Infra-red thermometer that can be uses for such propose. Measuring the sky temperature and comparing with ambient temperature is a good indicator of how cloudy is the sky.
The guys from http://tech.groups.yahoo.com/group/sgl_observatory_automation use Sensurion SFT11 conbine sensor to make dew heaters. See Sensirion page
Is available at http://www.adafruit.com/products/246
A good and cheaper alternative is DHT22 chip http://www.adafruit.com/products/385
This is a very convenient Temperature sensor. It has only tre pins (VCC,GND,OUTPUT). Connect power supply (GND,VCC) and OUTPUT to arduino analog pin. Its output produce 10mV/ºC signal.