Google labs just indroduced new way of writing in Arabic, its with تَشْكِيل now, all what you need to do is to write down in the box and then click on تَشْكِيل and thats it!
Click HERE to go to Google’s labs.
there’s also this cool (?) thing where u write in an english word arabic letters (cuz u forgot to switch language) but u still get english suggestions 🙂
try it !
try http://www.yamli.com its before google labs
i guess better late than never.. you just discoverd this!