The ARDrone is completely a toy. It has a range of about 20m before the video cuts out and the quad flies away by itself, crashing into a building or just falling out of the sky.
That said, you can make a long distance plane complete with video, gps and autopilot for $1000 - $2000. Depending on setup, flight time is between 15 minutes and an hour, with effective ranges up to 20km.
HOWEVER, very few people are capable of pulling off long distance FPV. Hell, most people fail at short distance FPV. The gear is hard, radio interference is a huge bitch (especially if you are near any urban area) and frankly, it's hard to fly a little plane made out of foam. They get knocked around by wind gusts really easily.
Any rotor platform (tris, quads, etc) are limited in range to 1-2km max, simply because you can't strap enough batteries onto the thing for longer range. Flight times are 15-30 minutes with current technology
Obviously, technology is going to continue to advance. And if the project is bankrolled by a government, all bets are off. But lone terrorists are going to have a hard time pulling off a drone attack. You can simply triangulate their signal (both the radio control and the video feed, which also usually includes a GPS downlink) and know exactly where they are transmitting from. Interfering with a drone is as simple as jamming the radio spectrum they are using...most drones (planes or quads) have about 10seconds before they crash into the ground when uncontrolled.
Lastly, carrying gear is really hard for any rotor platform. Max payload right now is a couple of kilos, usually a DSLR on a gimbal. These have to be lifted by octocopter setups (8 rotors) which absolutely kills battery efficiency. You get 15 minutes flighttime max, and the thing sounds like a swarm of killer bees.
They are even smart as real humans and don't care about their life.
The other point, terrorism is not abut effectiveness it's about publicity. Drones are not so much changing the game as it is right now.
If I was a secret service coordinator, I would be shacking in my boots when I think about how to protect the president when any enemy, whether they are a country, a terrorist group, or just a lone nutjob (and how do you even identify a lone nut job? The norwegian police pretty much admitted that they couldn't, after the debacle with the loser who shot up a kid camp), can make or adapt a drone and fly it from far away?
The president is safe now, because very few people are completely unconcerned with their own lives, but with drones we may never know who was really responsible for the attack.
Drones even have the potential to be more dangerous to political groups than nukes, because nukes are so difficult to make that the only entities who could make them are the large, advanced, countries that have too much to lose if they were ever used. Drones are much easier, especially as all it really takes to make a lethal killing machine from a standard drone is to strap a rocket launcher on one.
And yes, private people can buy drones (http://ardrone.parrot.com/parrot-ar-drone/select-site) for surprisingly little money (less than all but the cheapest computers) -- true they can't carry any load at present, but that is almost guaranteed to change in the future since drones have plenty of business usages, in industries such construction and transportation (and possible in the service industry, if the tacocopter takes of).
And if there is one thing a terrorist could get their hands on with relative ease it is rockets and even if they somehow can't you can properly do pretty decent damage just putting an AK47 on it.
There is a final, perhaps more sinister issue with drones -- unlike nukes you don't have much, if any, idea of where they came from; and if you don't know where they came form, how do you make sure that the enemy knows that you will retaliate? And if they don't know that, what is to stop them from using the nukes on you? Heck isn't it better, all things considered, that you attack first?