Daniel, just a question: you say that the bombs on 7/7 only managed to do relatively light damage to the train/bus. You don't need to do a lot of damage to a plane to cause serious trouble. If that "trouble" causes the plane to crash, the plot will have succeeded. Failure to bring down a plane will still be a (slight) succes, because of the fear it would spread. ("Next time it could be me, and they might succeed then!")
It seems to me that the original article was rather short-sighted. It only focuses on *one* type of explosive that someone said the plotters were going to use. The article then says that that type of explosive is hard to make. I'd say that you cannot conclude that the "myth" is debunked just because one type of explosive is hard to create.
FYI, the BBC had an interview with an explosive expert. He says that liquid explosives could take down an airplane.
"Dr Clifford Jones, an explosives expert from the University of Aberdeen, says even a small amount of liquid explosives carried on to an aircraft would result in a catastrophic explosion."
<a href = "http://news.bbc.co.uk/2/hi/science/nature/4780391.stm">the article</a>.
Without full information on what kind of explosives the plotters were going to use, it's pretty silly to try to "debunk the myth". But then again, the police are not going to release that information because it might help other would-be terrorists... I'll let a judge decide if it's a myth or not; at least (s)he will have all the information necessary.