You know those Kensington barrell locks that come on a cable?
They have a little prong on the back that fits in that slot while the lock is "open". Once locked down serious dammage would be neccessary to dislodge the lock. The attatched cable is affixed to or wrapped around a fixed object like a counter-strut or plumbing pipe.
Yeah, that's all it is! It's great though, the G4 has it too... I have some Macs in my High School, and we used to put a wire through the old iMac-grip on the top... but the safety-slot thing is way better! Even High School students can't destroy that