http://googleblog.blogspot.com/2009/07/should-you-spring-cle... indicates that solar panels mounted flat do indeed need to be cleaned, but panels mounted at a sufficient angle will be rinsed off adequately by the rain and do not seem to benefit measurably from manual cleaning.
Average soiling loss for tilted installations is around 5%, but it can vary quite a bit with location and rainfall patterns, so it is occasionally worth manually cleaning them.