Ice Lance and Frostbolt are both missiles, Ice Lance has a faster velocity than Frostbolt, Ice Lance can be cast immediately after the Frostbolt (near instantaneously with a /stopcasting macro), hence the Icelance will reach the target before or at the same time as the Frostbolt. Shatter relies on Nova being up when the spell LEAVES YOUR HAND, so even if the Ice Lance or Frostbolt (whichever hits first) breaks the Nova you still get the crit bonus because of intended game mechanincs.
Working as intended, the trick is so popular it has its own name (Shatter Combo), it can even be combined with a Water Elemental with a well timed Freeze to maximize chance of the Freeze not breaking.
Just about everyone knows this. No discussion needed. Thread closed. (But left undeleted for those who didn't know)