Visit our friends at Earth Electric for your additional solar needs. They offer Consulting, Installation and Residential Solar Inspections.
I have seen increases up to 20% after cleaning. They hadn't cleaned their panels since they were installed,  4 years ago.  After 4 months they were still maintaining this peak output.

This 20% increase is in line with research that has shown that in California, an average of 5% of your energy production is lost due to soiled panels a year.  Homes located near freeways and airports average a higher loss.
I am always asked how often panels should be cleaned.  I would love to come every month to clean your panels but it wouldn't be necessary or very honest.  It depends on your soil load. In most cases, once a year should be good. However,  a cleaning early in the spring and late in the summer could provide you with clean panels during key times of the year.  A spring cleaning to optimize the summers suns rays. Then another cleaning in the fall to remove soil that would greatly reduce your production when the sun is crossing the sky at a lower latitude.
Why clean your panels? Increased energy production is certainly the biggest reason.  Another to consider is that dirt has a way of etching the surface of your panels. It is just a good way to take care of your investment.  It may take a trip to the junk yard to see what a windshield of a car would look and feel like if it was left to the elements. Rain alone just won't take care of the dirt, especially in California.
Are you a participant in the California Solar Initiative? (CSI)  If you are, cleaning your panels twice a year makes sense and cents.  For every kilowatt hour you loose it costs you .39/kWhr. If you have a 10kW system and are loosing 5%, in 6 months you are loosing nearly $200.
408 799 2598

Check out what Google has to say about cleaning their panels. 
http://googleblog.blogspot.com/2009/07/should-you-spring-clean-your-solar.html