I've built a JK flipflop - specifically, the A version, from Redstone JK flip-flops - and it works - sort of. It doesn't behave as described in the wiki, and I'm not sure if the wiki description is wrong, or if I built something wrong - or maybe the Beta release changed something.
The wiki says that the type-A flip-flop is edge-triggered, and will change its output when the Clock goes from 0 to 1, or 1 to 0. For a Clock, I'm using a Switch on the side of a block (with three Redstone wires between it and the circuit, rather than the one wire in the diagram - for some reason, it breaks with only one wire between two blocks.)
When I flip the switch down, it transitions from 0 to 1 (unpowered to powered wire), which causes the Output to change, as expected. But when I flip the switch back up (transition from 1 to 0), I get no Output change (for any combination of J and K).
Just for fun, I hooked up a 5-Clock as input, in case there was something strange about switches on blocks. But again, the Output only changed when the wire from the Clock went from unpowered to powered, and not the other way.
So is that how a JK flip-flop really works? At least in Minecraft, not the real world. :)