Thank you, Kandinsky, for your link to "Southern Television broadcast interruption hoax." I thought it was fascinating, so I decided to do a full entry on it--as an object lesson on how to do proper research.
I read all of the references in the Wikipedia article (that I could get to) on it. Based on the transcript alone, it certainly *sounds* like a hoax... because any hoaxer would want the message to seem plausible on one level, while including an element of whimsy or absurdity so that we can be in on the joke.
About half of the references to the transmission label it as a definite hoax, while the others argue that while the transmission appears to be a hoax, the perpetrators were never identified or caught. (I can't get to what appears to be the most detailed account, from "The Daily Collegian.") Of course, if I were a prankster of this sort, I would try not to get caught, though I might have bragged about it anonymously years later.
This is one of those events that probably someone should have devoted more work on at the time.
Now, any careful researcher (as Collins should have been) can include the transmission in his books or DVDs, and even claim that it's ET in origin, but he would have to warn his audience that the transmission has been labeled as a hoax, and point them to the references, so that they can draw their own conclusions. In my opinion, it's misleading to present such evidence as genuine, allowing his audience to believe that it is indeed an ET broadcast, when in fact it has been given a cursory investigation and is alleged to be a hoax. At best it's a mystery, or "unexplained."
This event seems to be one of those litmus tests designed to expose a person's ideological orientation. The non-believers in Ashtar (the majority of the educated population) will immediately believe that this was a silly prank. Those who want to believe in ET will label this as a mysterious event that suggests that ET is out there.
What do I believe? Well, I seriously doubt that it's ET. A commenter on one of Collins's boards pointed out that the message is remarkably similar to the stuff in Andrija Puharich's book on Uri Geller (which I read at the time)... and this lends the broadcast credibility. I think it's the opposite. I think that even Geller himself implicitly disavowed Puharich's book. Talk of "Ashtar" was very much in the air in the mid-70s. Ruth Montgomery wrote a number of accounts about Ashtar during the time, and these channelings helped form the basis for the "axis shift" predictions that came later. I think that the broadcast has to be viewed in this cultural context. Because the perpetrators (be they ET or college kids) were never caught, I am leaning toward the possibility that the broadcast was one of those weird, Fortean mysteries which may never be explained, but its association with the UFO cult beliefs of the '70s tends to discredit it.