The most common claim for these "alternate scenes" in Star Wars is that they were shown on some obscure TV broadcast. But the fact is, none of these broadcasts were really obscure. Star Wars airing on TV was always a big event, especially in the early days, and hundreds, thousands of people taped them every time. And the fact is, out of all of these taped copies of all of the times that Star Wars ever aired on TV, none of them have ever had any alternate footage in them. Not a single one, ever. We've had people going over these videotapes with a fine-toothed comb. There has been an alternate soundtrack aired in the UK (mono), but the footage itself has always been the same.
I will be more than happy to eat crow if someone can produce real evidence (i.e., actual taped footage) otherwise. Somehow, I have a really strong feeling that's not going to happen.
--SKot