AC polarity

Actually AC IS very asymmetrical with respect to ground, although Clueless is obviously correct that if you ignore ground and only check between the two AC terminals, you can't tell which is which. In the vast majority of cases, though, the "hot" side (black/narrow) will be around 120V from ground and the "neutral" side (white/wide) will be very near but not exactly at ground. This has both safety and electrical implications. You can usually grab the white side without much shock, whereas the black side will get you every time (no, I'm not recommending you try it!). And in the electrical case, it can be the source of ground loop problems if different components are hooked to the power source in different ways or grounded differently.

It doesn't mean anything current-wise, they only went with one narrow and one wide blade so plugs could be oriented and not inserted backwards. In things where they use nongrounded plugs, like electric drills, TV's, some pieces of audio gear, etc, they still want to design the piece to protect you from the "hot" side as much as possible, which means orienting the plug. The current through each blade is identical no matter what.