Is it because, the CDDL does not let its derivative to be released under GPL? Hmm., I think I'll look up the license., this stuff should be in the conditions of distribution part.
Legal definition of the term "derived software" is often confusing. Let me put in simple terms.
Code from CDDL and GPL *cannot* be combined together. A GPL'd code cannot even link to a library licensed under CDDL.
How ever it is possible to dual license (GPL/CDDL) the code. One good example is Mozilla project's triple licensing scheme (MPL/GPL/LGPL).
-- Anand Babu