There are already 5 books in the series and a new one "The Wimpy Kid Movie Diary" is due out March 16, 2010. This book is a behind the scenes look at the making of the movie. The transitioning of the first book onto the Big Screen. It is written by the author, Jeff Kinney, and looks to be a great addition to the rest of the books in the series. I am definitely going to get it for my collection.
Then there is going to be one more book, so far, coming out next fall. That's all I know about now.