I didn't always play Mage I just rerolled one for WotLK.
I think "Rolling" ignites were just the damage stacking on top of the previous ignite causing the old ignite damage to stack with the new ignite damage.
I recall hearing that Rolling ignites did not exist in TBC.
Now I'm FFB fire spec and from what I see it looks like ignite is rolling again. But since I don't really have past experience with it, I don't know if its rolling again...