Someone over at http://www.anderoid.com/~seander/archives/000111.html noticed a very interesting trend:

Google's index should be growing exponentially to keep up with the growing size of the web.  Google's main page indicates that it searches 4,285,199,774 web pages, and this number has remained the same for the last year and seven months, since around February 8-14, 2003

and they also have a guessplanation for it:

I bet they are suffering from a 4-byte limit with their URL identifiers.  With 4 bytes, which is the natural word-size for the inexpensive ia32/x86-compatible processors they are using, they can store 32 bits, and that means 232 different values, or 4,294,967,296.

Interesting..