This is to announce the immanent support for Arabic and other joining scripts in the next release. You will be able to select between letter spacing in fonts/scripts (well you could before, but it broke your output) and get successful spacing between letters that aren't joined.
Unfortunately, due to the limitations of xetex spacing won't happen between a right joining character and another character if there is a diacritic character between them. This is suboptimal but not a noticeable problem, I hope.