I don't know if they're available in the USA, but in Canada you can buy a two-prong male to three-prong female current tap (with a tab on the two-prong end that is supposed to go under the screw on the receptacle cover). With that tab removed and the power brick plugged into the female end, I've found you can plug the brick into any receptacle no matter how deep. Don't worry about the lack of ground, the brick itself is two prong.

I think the brick includes grounding on the metal post holding the plug/cord in place - note that Apple's extension cord does have little metal springs that contact this post. Of course the 2-prong plug does not have a ground plug, so obviously the ground attachment is not strictly necessary.