I recently got a Mac, and I'm having trouble rendering opentype Arabic fonts. Apple native fonts render OK, and I can write Arabic with Pages, for example. But I can't use opentype Arabic fonts, there are no ligatures. So, SIL's Scheherazade AAT font works, but the Opentype version does not.
And Arabic opentype fonts either render clumsily in PDF in the native PDF viewer (generated with Xelatex). The same pdf shows OK when using some external PDF viewer (such as Texworks). (See the two attachments). I don't know about Adobe Reader, I thought about installing it to see if it could make a difference, but deemed it too bloated (400mb + just for a PDF reader), and I don't even know if it will make a difference.
I installed the most recent updates, I'm afraid there was some bug with Opentype fonts and PDFs in Leopard, but other opentype fonts are OK.