This is a topic that was brought up on The Dragon's Flagon: what happens if you are struck by something gigantic? In Second Edition, the giant thing provokes a saving throw (usually vs. breath weapon) to see if you can fling yourself out of the way. So far so good, right? But what happens if that giant thing is tossed by, say, a giant? Well, now it's an "attack" and you don't do anything. The giant rolls to-hit and the boulder either strikes you or passes you by.
Why is this unsatisfactory? There are many reasons. The first and most obvious was brought up Mr. Treasure, and that is this: armor can't protect you from boulders, axes the size of a bus, or hammers with a 15' long haft. These things are less like attacks and more like traps. The giant doesn't have to work very hard to try to hit you, he only needs to pass his weapon into roughly the same space that you're standing in.
I'd take the attack roll for throwing objects. Perhaps you're struck a glancing blow by a boulder that's tumbling towards you, saved by your armor. But no one who is struck by a giant's hammer, glancing or otherwise, is going to be ok.
Eric suggests a saving throw to avoid being hit. I think that's a good idea, but it does reduce a combat action to a "sure thing" every round for the gigantic attacker. He will be scattering the party hither and yon with every step, sending folks flying or diving for cover. Maybe that's good! I don't know, I haven't seen it in play yet.
We are certainly going to try it out and see how it fares. It may be too much, it may be just right. Either way, it is an excellent suggestion because there's just no way that I can see a man being hit with a bus and standing there unharmed, no matter how good his armor.