Technically, if you're willing to risk damaging it, you can force a regular USB device in there, but it sticks out at a weird angle because of the notch.
That's an example of lock-in that makes absolutely no sense. Why would you do that with USB extension cords of all things? The only reason I can think of--it physically locks the cables together better so it doesn't pop open in the middle of the extension cord where it's difficult to reach and reattach.
I'm pretty sure they do it because the keyboard can't source the required 500mA of current. They "proprietarize" it in order to keep people from plugging high-current devices in.
The keyboard itself contains a 2 port USB hub (regular ports, btw, with enough juice to power a Wacom tablet and charge my phone at the same time), so I'm not sure that makes sense.
I don't know about the current systems, have been wireless for more then 4 years now, but the original iMac had non-standard USB usage because Apple wanted the "power" button in the keyboard, while the USB protocol did not support this usage, at least it didn't at the time; I don't know if this has changed since then.
Here's a picture: http://imgur.com/Aweqo.jpg
Technically, if you're willing to risk damaging it, you can force a regular USB device in there, but it sticks out at a weird angle because of the notch.