The Pammac English Arabic Dictionary is the only 100% English-to-arabic dictionary.
It's database (almost 40Mb) contains more than 80.000 terms and expressions and is the only product for the iPhone/iPodTouch that perfectly renders Arabic text. It has no competitor.
You don't need an EDGE/3G/WiFi connection to lookup anything... Everything needed is already inside the application. Better impossible.
Give a try! It's very cheap!