I've posted a lot here about Arabic diglossia and the various spoken Arabics, so here's another entry for the files: The Economist's language blog, "Johnson," offers "A language with too many armies and navies?" Note, too, the comments section,where Arab readers argue against some generalizations about mutual intelligibility among the dialects.
The column also links to a paper by Tunisian linguist Mohammed Maamouri on "Language Education and Human Development: Arabic Diglossia and Its Impact on the Quality of Education in the Arab Region." (Summary page at the link; full PDF here.)
By the way, I'm in the midst of reading final galleys for the Summer issue and blogging may be light today and tomorrow.