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?