I've had a cable lock cut while shopping. What a pain.
So, if I'm concerned about security, I use the Kryptonite NY lock. Still not impenetrable, but generally requires it to be cut, either torch cut or grinder cut.
Amazingly enough, this image was on the Abus bike locks website; the annotations are mine. The way Abus locked it, the folding bike could be stolen with a hex wrench.
Yeah, same thoughts with my Bike Friday. For the ambitious thief, it would be easy enough to remove the handlebars, and potentially slip the lock down the frame and around the fork (note the Birdy uses a suspension fork).
I normally lock the Bike Friday up through the rear triangle, cutting a diagonal through the triangle and wheel so I think it is more or less trapped by the tubes. The cable guide is also a potential target for the lock, but not as big as the triangle on the Birdy.
Each bike is a little different.