I have been increasing reading 100's of posts about problems with Google indexing or de-indexing pages. Some people are having difficulties getting pages indexed quickly and others are finding that a successful site suddenly and inexplicably has large numbers of pages de-indexed. Firstly YOU ARE NOT ALONE. This appears to have been happening increasingly and it may not be because you have done anything wrong so don't beat yourself up about it. The truth is that no one, except for Google can explain these increasing indexing issues. Secondly now is a good time to remind yourself of the "Quality Guidelines" created by google to help google bots find, index and rank your site. You can view these here: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=35769 It could be that you need to revist your site structure. Above all don't panic, my experience of this issue in the past is that Google sorts out the problem, but it's fustrating in the short term especially if you are looking traffic and ultimately revenue.
Pretty good advice. Most folks over react to changes in Google instead of waiting for the dust to settle.
This certainly happens due to lack of basic knowledge! People are mostly concerned about the results and not on the metrics!