The sequals/prequals actually have quite an interesting story of thier own. Frank Herbert origionally wrote six, and the common misconception is that they were 2 trilogies. In fact, Books 1-3 was a trilogy, book 4 (God Emperor of Dune) was completely standalone, and books 5-6 were the first two books in the second, incomplete trilogy.
Brian Herbert, Frank's son, is a writer in his own right, and based on his father's notes, he and Keven J. Anderson (known for his Star Wars books), first wrote a prequil trilogy, the "house" trilogy, that basically set up more fully for the events of Dune.
They are in the midst of writing a second Prequil Trilogy about the Butlarian Jihad, which was basically the war against AI that happened 10,000 years previous to Dune. Because of this war, all "thinking machines" are outlawed, which allows for computers, as long as they do not mimic the patterns of the human mind. Thus, there are com-links and such, but no robots, etc.
Anyway, I actually went to a book signing a few years ago for Dune: House Harkonen (book 2 of the prequil trilogy), and Brian basically said that once he and Kevin finish the Butlarian Jihad Trilogy, they are going to go back through his fathers notes and attempt to write the final book in the series- Book 7. The notes left a lot of things ambiguous, but there was definite direction going on.
Oh, and I've read dune at least 30 times, and have about ten copies of it in three different languages (english, french, and spanish). I love them books.