The people that did this are a bunch of cowards who somehow think that taking innocent lives will somehow help to prove that their ideologies/beliefs/protests/whatever deserve to be taken seriously. Nice going there, idiots, you've definitely got my vote now!
A chilling thought struck me earlier. What if this was intended to clash not with G8, but
Live8? All of the attacks took place in the vicinity of Hyde Park (take a look at the BBC's
map of targeted areas if you're not familiar with London). Maybe someone in the responsible organisation accidentally said July 7th instead of July 2nd. Maybe the bombers misheard the command.
It could have been much, much worse. Now that's a frightening thought.