It's depicted as an apple, but is simply referred to as "the fruit of the tree of the knowledge of good and evil". So you can't pin it down except that it came from a tree so strawberries are probably safe. It's interesting to note, though, that taken literally the design for humans was to have them totally amoral...
As always incitatus is right. Still, the question is interesting. Since apples are so rare in that part of the world, it is plausible that the most valuable fruit was picked to seduce Adam and Eve. If the whole story had taken place in Western Europe, the fruit would probably be the orange, or the banana. After all, even in paradise there has to be a hierarchy ;(