I've done quite a bit with another complex script, Devanagari for Hindi. While
not being RTL, it also has a number of interesting problems in Flash.

My solution was to compose my text in an external text editor and save as
unicode (UTF8), then import to Flash using either the XML or LoadVars classes ?
depending upon my purpose for each little part. I'm on a Macintosh, so I use
TextEdit, but my Hindi professor is on a PC and she uses something called
Takhti. So as long as the applications supports unicode it shouldn't matter
which platform you are on.

As Mohammed points out, Flash doesn't break the lines properly. I had to write
my own code to do line breaking. I am happy to share it, but will be away from
my home computer for the next two days.

Now one problem I've found is that if you embed the fonts in the textfields,
Flash will not properly render the text. The characters will show, but they
will be in the wrong order or placed incorrectly. This is with Hindi and Greek,
but I'm sure it will also apply to Arabic. You will probably just get the
isolated forms for each letter or something like that.

My solution was to not embed the fonts. This requires that the end user has
unicode support and a font installed for the language. Of course between the
Mac and PC there are different fonts with different sizes, etc. You just have
to take that into account when you design your page.

Good luck.