Created attachment 59712 [details]
Screenshot showing incorrectly rendered combining diacritics
Arabic has a number of vowel markers that essentially function as combining diacritics: they must be placed over or under the letter they are associated with. On OS X, in the Sans Mono series, these marker are not combined with the predecing letter but take up horizontal space.
The attached pdf file contains two screenshots. The first shows a word without vowel marks in the top line, where the letters are properly connected, and the same word with vowel diacritics, where the letters are not connected. The second screenshot shows the same file on GNU/Linux, also with DejaVu Sans Mono, where the diacritics are displayed correctly. (Well, there's still one minor issue, but I'm pretty sure that's a bug in Emacs.)
My apologies for the poor quality of the images, I had to combine the two pngs into a single pdf file because Bugzilla doesn't seem to allow me to attach more than a single file. If you zoom the pdf out a bit, the fonts will look better.
Note: the problematic screenshot was made with Safari under OS X 10.7.3. I have been told that the problem also occurs with TextEdit.app, but unfortunately, I cannot test this, because on my machine TextEdit.app does not allow me to select any of the DejaVu fonts, for some reason beyond my ability to grasp...
Note also that the Serif and Sans series do combine the diacritics correctly (as tested on Safari).