The "base" for arabic in windoes has been there for a while now, but OS X is a new product. Arabic exists in OS 9 as a language pack which you can download. If you dont have arabic for OS 9 I can try to see if it is as a specific link that I have so you can download it
I think the fonts do exist for arabic (as part of unicode) but a method for input is not there. The same situation exists for many languages like greek and russian
I am sure we will see some sort of input method from apple soon (or from some developer anyway

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Admiral