my old jetta had a rough time starting at anything below -15C... thats with cold start pulled all the way too..
it had new rings put in, but pistons were a bit sloppy. glow plugs worked.
i think its safe to say that white smoke could mean that you may need new rings or compression is low. white smoke might also be bad injectors though... one thing is for sure. an engine with low compression will not start without GP's. it seemed as though the motor would not even attempt to fire.
oil makes a huge difference too. thick oil will reduce cold cranking speeds by a whole lot. i think everyone here will agree that switching to 0w40 is a great idea in the winter.
my first winter with a diesel, i went to a buddy's house, and that night the temps dropped to -30 or -35ish. the battery was alright, but the engine was cranking over very slowly. i had to leave it there and wait for a boost. even with a boost it cranked pretty slow. (the block heater was connected, but i think there was something wrong with the wire because it did not seem to be working :x )
needless to say i got pretty pissed with diesels, but i have grown to love the sound, and the mileage

. make sure you have a 1000CCA battery if your car has low compression, and check the GP's.