I might be persuaded to let a PC take 10 on an attack roll, but only if it were the first attack of the combat. After all, it lends some credence to movies/TV shows/ books/whatever as the sort of "lazy fighter"- look at Indiana Jones and the Last Crusade, when he's in some middle-eastern town and a guy comes out and does an intimidating Perform: Weapon Drill check with his Scimitar, showing his amazing prowess. Indiana Jones simply takes out his gun, shoots him, and walks away. Doesn't look like he was too worried about that one.
That's an example of someone using it on an opponent with a low AC (or at least one he was easily able to hit with an average roll). There are lots of times in other sources of characters with low attack bonuses/enemies with High AC "taking 10" on their attack rolls- I can't think of any specific examples, but picture a guy who thinks he's the greatest fighter in the world, and he sees an opponent. He simply walks over, and lazily swipes at the guy, expecting his high attack bonus to show his opponent who's boss. His sword, however, simply taps against the guy's heavily armored skin, who then grins and whups the hero's butt.
Basically, there's lots of instances where characters in stories are incredibly confident in their ability, and are able to "take 10" on their attacks. However, it almost always only applies to the first attack, where either their opponent is felled by such a great blow, or the attack misses horribly because of the opponent's battle prowess. In the second case, or even in the first case once the attack is over, the battle generally becomes very hectic and distracting, prohibiting anyone from "taking 10". So if it were in my campaign, I'd let the PC take 10 on their first attack. Who's to say the goblin doesn't have a one-shot item that automatically bumps up their AC for the first attack? ;-)