Check out this new article from the Daily Illuminator that Steve Jackson Games posted:
December 26, 2006: Heartlessly Teasing The "Casey & Andy" Fans
We are sorry to have to tell you that this book does not actually exist. Not . . . quite. There is no giant hardback GURPS Casey & Andy book. Darn it.
There is, however, a real GURPS Casey & Andy, written by David Morgan-Mar. It's an e23 PDF. And until Andy starts updating again, you might as well create your own adventures and make the characters do whatever you want. Muahahaaa!