In October of 2011 Google announced that signed-in users will, by default, be routed to the SSL version of Google Search (https://www.google.com).
What is the impact of this change for webmasters?
A web site accessed through organic search results on http://www.google.com (non-SSL) can see both that the user came from google.com and their search query.
However, for organic search results on Google’s SSL search, a web site will only know that the user came from google.com.
This is bad news for anyone who wants to know what keywords were used to find their site on Goggle.
Over two thirds (69%) of my Google Analytics Keywords now show up as “(not provided)”. This completely cripples my ability to tell how people are finding my site.
Here is a good video I found on SEOmoz that goes into more details and explains how it works: