Save vs. ginormous attack!

I've been thinking lately about the fundamental differences between types of attacks.  Specifically about the differences between attacks by opponents of similar size, and those by massively larger ones.  Armor certainly protects against the former, but how much does plate armor really matter when a frost giant swings a giant-sized axe at you, or when a chunk of catapult shot the size of a small keg comes hurtling your way?  In other words, is a standard attack roll against the target's AC really the right way to resolve such an attack?

I'm thinking that a saving throw might model the situation better in those cases.  Attacks of such awesome magnitude are going to smash through armor like it was tinfoil; the target's only hope is getting the hell out of the way before it crunches him.  That feels like the province of saving throws to me.

Many really big attacks are actually more like area attacks than standard melee maneuvers.  A giant doesn't just swing at one man-size target, he sweeps with his weapon, battering aside multiple foes.  All you PCs standing in a line in front of him?  Save vs. breath weapon!  Nope, I don't care how much metal you've wrapped yourself in.  Miss your save, and you take the damage listed for the monster's attack.

I'm leaning toward a save vs. breath weapon because it's similar in that it's a save against something hurtling rapidly at you, and because it's a fairly difficult save, with fighters being best at it (out of the human classes, at least.)  Cover might provide a bonus to the saving throw, just as it penalizes the attack rolls for missile fire.  Dexterity bonuses or penalties might also be applied.

Some other attacks that might be in the "save vs." category rather than roll-against-AC are hurled boulders, huge bite attacks like dragons and purple worms, and the trampling attacks of large creatures like elephants and rhinos.  Not every attack from a huge creature fits this model; things like dragon claws or the stings of wyverns and purple worms are more precisely targeted, and should probably be made in the standard fashion.

From a game mechanics standpoint, some of the effects are:

Making armor irrelevant in these cases, and thus making other factors such as mobility relatively more important.
Making the outcome of an attack hinge on the skill of the target character rather than the Hit Dice of the attacker.
Allowing certain large or huge attacks to strike multiple targets under certain circumstances, making tactical movement and placement important.

I haven't crunched the numbers yet, so I don't know how great the impact would really be, but it has a decidedly different feel to me.  Is this a good idea, a bad idea, or simply a pointless change merely for the sake of changing something?  If you have any thoughts, feel free to weigh in.