I'm sure that's where the Wiimote was supposed to hook on. So when you're using the Classic Controller, you can keep the Wiimote on the back of the controller, rather than having it lying around. I haven't got a Wii, so I have no idea how they would connect, but hey, that's just something I read about a long time ago. In fact, this is the first time I've ever seen a picture of the back of the classic controller.
Vagrant, thats exactly what I'm talkin about. They're definately on the one I picked up at Wal-Mart last night. It didnt come with any sort of connector that I can use to hook it up to the Wii-mote, but I had that thought too. I really have no clue what those slots do.
remember the ext port on the bottom of your SNES. it was so they could hook up the hard drive and the cd rom unit. then they shelved it. its likely left over from them having planned to attach it to the wiimote, and they didn't want to redesign and re-cast the thing. leaving it open to some possible future use. there are absolutely no electronic components to it though. it is for fastening only.