like IMEI numbers for phones, you can do lots for to deter criminals if the thing is worthless once stolen. Breaking a frame on the crossbar and hinging it on the crankshaft means the only way to got the bike of an immovable object is to trash the bike.
 This would be a really good thing for bike touring, where insurance demands that you use a lock on the specified list. All of these weigh a couple of kilos and I'm loathed to take any of them given how little the rest of a camping outfit weighs. Memo to self...invent this and got it on the sold secure list.
It could event have a swappable lock mechanism so you don't have to throw the whole thing away if an exploit is discovered and you could replace it with a lightweight pin when security wasn't required.
Puma have created a destroy-on-theft bike (mid june 2010!) - the disko. I guess if you break the wee wire, it folds up if you try and ride it - unless you have a bit of string to hand...