Google's PageRank has been a mysterious and sought after value by SEOs for a really long time now. One of the ways Google secured the value (1 through 10) was through something called a checksum. Without the checksum, you are unable to check your PageRank, requiring you to use the Google toolbar. Now that this is out for the public, I am sure Google will be changing it around a bit.
I first found this by way of a comment left at this site by a reader. The person left a link to a thread over at mobileread.com, where the member try to break the checksum. It seems as if they don't understand how valuable this code is to SEOs until after they broke it.