i've always wondered about this too. back when i was a young noob, flying buffs all the time, i used to bomb the HQ a lot, and one time there was a 109 trying to get me, just as i was dropping, it looked as though the bombs were going to hit him, i dont know whether he missed them, or whether they went through him. i still dont know the answer, dunno whether its possible or not.
however i have killed many people using rockets, and have been killed TWICE by GTR in the 8" guns, which miraculously hit me, while i was in the air! it happened TWICE on seperate occasions!!!!! nearly pissed myself laughing both times lol
but yeah, i would be interested to see the answer to this. i think i've dropped bombs on friendly bombers before by mistake, but they did no damage, probably because of friendly collisions