@ John 186
Aircraft radios are not difficult to interfere with, but it's not really *that* much of a problem for a number of reasons:
Most airports have more than one frequency - so they can change to another if there is interference (or emergency communications) on one.
There are written-down procedures for dealing with a complete radio failure - which may involve things like flying a certain pattern the radar controllers will recognise, or setting a special transponder code which will also show up on the radar screen.
If a terrorist wanted to interfere with aircraft transmissions, it's easy enough to do, they could just buy a transceiver for about £100-£200 from one of the online pilot shops - many pilots carry them as a backup, but inevitably some have fallen into the hands of "pranksters", so the legitimate users are certainly aware of the possibility there's a nutter or terrorist on the other end.
It's fairly easy to recognise someone on the radio who hasn't been trained in the language and mannerisms and can therefore be disregarded - inconvenient, but not dangerous; plus, the control tower can get a bearing accurate to a few degrees of the direction of any transmission (google "VDF", probably how they found the equipment in the article). Police have this capability too, I think.