Originally Posted by KnhoJ

Titanium's not that bad. I've even cut it with hand tools, a pipe cutter and a hacksaw. It's softer than steel unless it becomes work hardened. You've just got to be aggressive with it, and not try to make a nice gradual cut, or it will unsharpen your pipe cutter blade. (been there, replaced that blade) But what's really fun is grinding titanium! Once you've seen the shower of sparks thrown off by titanium, you'll never call anything else white again. Just don't breath the ground titanium dust, you'll regret it if you do.
Stellite would cost a fortune to make into something like a lock. I've heard that some of the alloys can technically be cut with carbide tooling, but that doesn't last long. It's generally machined with abrasive surfacing tools. Takes forever. And then there's the usual issue with really hard metals: Whack a good cutting tool with a hammer, and it will splinter. Hardness involves some balance between brittleness and softness. Cutting tools fall much further over on the brittle end of the spectrum than is practical for a lock. A lock as uncuttable/brittle as a drill bit would suffer the same fate under a hammer.

Stainless might be a good compromise, even good old 304. It's ductile enough to form and although it's a pain to cut it's not prohibitive. But the thermal conductivity is so low that it'll heat up locally and work harden in an instant if you make a mistake cutting it. Try drilling a 1/4" hole in 12-18 gauge stainless at high speed and low pressure and you'll see what I mean. You'll get one severely dull bit and a blue dimple in the sheet metal that's nearly impervious to most drills. Abrasive tooling isn't exactly effective, because the heat produced means that the abrasive is always scraping through hardened material.
Maybe if a stainless u-lock had multiple small cavities running through its length, those would serve to collect work-hardened chips, which along with the multistep cut would do terrible things to a cutting tool edge. Not to mention, if someone went after it with an abrasive cutoff tool, the multiple heat-hardened sharp edges formed by the cavities in the stainless would cut down the abrasive a lot faster than the abrasive cut the lock. Empty spaces would save some weight as well, which would be important since stainless weighs a fair bit more than steel.
But at the price necessary to pay for additional design, material, labor, and tooling to deal with stainless, it would be tough to sell enough locks to pay off the original investment. If I get really bored someday and I'm feeling a little masochistic, I might make up a stainless U for my New York lock. But probably not, stainless is a complete headache to work with, insanely heavy, and if someone's got a sawzall they'll be able to get through any bike rack in under a minute anyway. Such is life.