OK, since no one has posted for a while I'll sum up:
91% of you have heard about it before, only 9% (2 people) haven't.
A lot of people have seen the MythBusters episode, some had heard of the Mythbusters episode, some have seen it in some video clip on the internet.
However, the phenomenon seems to have become a matter of interest post 2000.
Now, the reason I asked if you knew about it was
a) I wanted to know if it is really so well known or if I just know the "right" kind of people
b) I wanted to know if anyone has heard of it before 1996/7
But why did I want to know these two things? Because I suspect I started it. At least I accidentally discovered the mentos fountain when my class and I came back from a school trip. I had some rolls of mentos and was just munching away on one when a friend sitting next to me opened a fresh bottle of coke (I think it was not coke light, though). I said "Want a mentos?" and without waiting for an answer and without thinking what I was doing I dropped it into the bottle. It was very surprising. I'd never have thought it would gush like that. Of course I wanted to create a little foam, but not a huge fountain. He was drenched in coke afterwards, another friend of mine who sat in the row before him had coke down his neck and back, I was more or less dry (
I hadn't heard of it before, and only some years after that I began to hear about it again from people that were not either on that trip or at my school. Since none of you seem to have heard of it before early to mid 2000s, I guess it could've been started by me and then spread over the internet.