I know this thread is old but I feel compelled to add a comment, as I'm sure others will happen upon this site -- and this thread -- just as I did.

I'm 50 years old and grew up in the Chicago area. I am of above average intelligence, at least. I have always said and heard "all of the sudden". It was the few times I heard "all of a sudden" that caused me to Google the prhases and consequently, end up here.

What I am most amazed at is the absolute certainty with which some of the people who have contributed to this thread speak on the subject. Just because you believe it to be one way or the other ("a" vs. "the") does not make it so. I found myself saying "Who are these people that they are such an authority on the matter. As I kept reading, it became apparent that they are only an authority in their own mind.

Kudos to Dan, who was right on target with his take on the matter, when he said, "The bottom line is that it is an idiom. Idioms have established societal rules, but no grammatical rules. So, if people started saying "all the sudden" all of the sudden, then that would become the accepted idiom!"