Directly from the patch notes of 3.0.2 Spell casting and spell channeling pushback has been changed to the following:
* When casting a spell:
o The first and second hit will add .5 secs each to the cast time.
o All hits after the second will have no effect.
* When channeling a spell:
o The first and second hit reduces current duration by 25% of total duration each.
o All hits after the second will have no effect.
So basically you'll get hit twice, and continue casting like normal regardless of how many times you get hit after those first 2. Same goes for your Channeling Spells except your spell will be reduced to a total of 50% instead of just completely interrupting it.
Edited, Dec 5th 2008 2:28am by Joobishwun