Didn't think this was supposed to be possible but I just raided an ally! With a sly grin counting my loot, I get a "WTF!" chat from him on our old faction chat program (we used to be in the same faction). So when I apologize and we exchange ID to make sure it doesn't happen again, we both get the 'request failed - you are already allies'.
This happened to another 2 faction members before..in fact the attacker is now known as Mr. Friendly Fire lol. Didn't think this was supposed to happen...