a) buying a few hand grenades and premade multicopters? Hobbyists in Germany have already multicopters which are GPS pre-programmed so you don't need any pilots to control them during the mission.
b) creating thousands of pounds of explosives? Try read the detailed diary from the 2011 Norwegian terrorist to see how much time and trouble it took produce his bomb: way more time and effort than most would think.
The next 10 years will make multicopters much more accessible to non-geeks, but "thousands of pounds of explosives" won't be any easier than today. New tech gets old very quickly.
Why do you need multicopters? Terrorists already can (and some have) plant small timed bombs in crowded areas. I don't really see much difference other than that they can target more secure areas. This would be useful if they wanted to attack military targets, but since their usual goal is civilian casualties, they can by definition already get to places civilians congregate.
I have absolutely no idea where where I can purchase a live grenade. However, living in a rural area, I'd have no trouble finding people who know how to make explosives from common (and cheap) farm materials. Hell, I was making blackpowder when I was 11 years old.
Now add to that the fact that in the US I can buy a used car or pickup truck cheaper than I can buy a cheap GPS-equipped model helicopter and I definitely know which option I'd be more worried about.
Hobbyists in Germany have already multicopters which are GPS pre-programmed so you don't need any pilots to control them during the mission.
Hobbyists everywhere can have this - Arducopter (DIYDrones.com) and other projects have had this for years. Arduino + compass + GPS + accelerometer + altimeter + optional XBee radio (~2km+ range on some models) + autopilot software... preprogrammed routes, live telemetry and course changes via Xbee and ground control software.
a) buying a few hand grenades and premade multicopters? Hobbyists in Germany have already multicopters which are GPS pre-programmed so you don't need any pilots to control them during the mission.
b) creating thousands of pounds of explosives? Try read the detailed diary from the 2011 Norwegian terrorist to see how much time and trouble it took produce his bomb: way more time and effort than most would think.
The next 10 years will make multicopters much more accessible to non-geeks, but "thousands of pounds of explosives" won't be any easier than today. New tech gets old very quickly.