This thread has forced me out of lurking and in to posting.
I just wanted to say that this particular type of threat has been extant as long as there has been a radio control aircraft community.
The problem with the 'drones' that are proliferating in the civvy world is that their distribution is via many many streams.
Radio control aircraft and their associated parts are generally sold through relatively few outlets and to relatively few people so a fairly decent community generally grows up around a good hobby shop. These guys do look out for their own, and have even been advised by the Police in the past about looking out for suspicious behaviour and persons. This doesn't mean there aren't ways in which people could purchase materials or equipment to make RC aircraft unnoticed in the past, and that's even more the case with internet outlets.
This sort of attack has been documented in the US and in Europe so it's not unheard of:
Students 'planned terror attack using remote control planes' - Telegraph
Terror and toy planes ? not so remote ? CNN Security Clearance - CNN.com Blogs
Remote Control Plane Plot: Massachusetts Man Indicted - ABC News
Seems in all cases that it was the observations of the people that uncovered the plot, rather than anything specifically related to the use of RC aircraft.
Would these plots have worked? Maybe. Lots of ifs-and-buts on the technical side, but if the aim was to spread terror, then yes, they most likely would have worked depending on the target.