Damn my players making me come to this forum to check on another post. I've been reading all the fun threads all night but none are as epic as this so far!
Lots of great points but I think the original intention was for it to be covered under the total defense and similar actions/feats as people stated ad nauseam earlier.
As far as a readied action, it states clearly that the readied action takes place before the action that triggered it occurs.
So you could not knock the arrow out mid flight just like how you cannot knock out a fireball mid flight.
Could you block it with a readied action by casting Wall of "name your element here"? Sure.
Can you attempt to disrupt the spellcaster? Sure.
"Closing the door as the archer fires" as someone stated earlier with a readied action? Sure.
People can have house rules and do whatever they want. But the slippery slope such a precedent can create will probably lead someplace dark and time consuming as it is applied on other ways not originally intended.
Kudos to the last example of total defense. As far as some examples of Aragorn knocking away the knife mid flight, it's perfectly explained under combat expertise or total defense. Just as AC represents a blend of luck, dodging and armor and HP's don't represent things actually hitting you and causing physical damage with each one lost. It's all about the presentation.
As someone said earlier, if you must put it within the scope of the rules, if the total defense maneuver bonus on top of the AC is what caused the arrow to miss, then why not say "You deftly swat the arrow to the side with your defensive parries".
Pick your poison.
Now snatching a potion out of mid air tossed between two enemies, that's the topic of another whole discussion. :)