I have been a locksmith for 10 years now, and get this question almost daily. The strands in the armoured cable make it extremely difficult to cut- they move and flatten and bend instead of holding still for bolt cutters. that doesn't make them impossible to cut, and the padlock you use to join the two ends needs to be of the highest quality.
in the end, if a thief REALLY wants YOUR bike, they're going to get it. a friend of mine used a pair of handcuffs (his dad was a cop) and a beat up old shwinn (or something) when riding downtown, never got stolen. finally rode his 'A' bike in to work one day, used 3 of the highest quality locks he could find and the guy that stole it used a reciprocating saw to cut the sing post he was locked to.
if your bike is that important, find a safe place for it. make friends with your local security departments and shipping-receiving offices, and have them stow your bike for you while you run errands or work.