I wrote about this topic last year, but it seems to have come to surface - the fact the Google may have hit their limit on the number of pages they can index.
As you probably already noticed, if you watch the web closely, Google doesn't seem to have performed a major update of their index in some time. There is an idea out there that illustrates why this may be so.
The idea goes something like this: The way Google was built was to use a unique identifier called a PageID which is used to categorize the web. Each page gets its own PageID. The problem may be that Google's PageID system may have only been built to assign a maximum of 4.2 billion unique PageID values (based on the math behind the ID value). Since they have been showing over 3 billion pages indexed since November, it is safe to assume that they could be nearing that 4.2 billion mark.