Of course, we'll never really know. One of the hallmarks of a terrorist attack is that nobody really knows for sure exactly what targets were intended, or the intended results—not even the terrorist leadership. Too, we can't know the full ramifications of the timing—just the G8 summit, just the announcement of the winning Olympic bid, just the election of an uncompromising hardliner as Iran's new President, triggers I can't think of off the top of my head, or some combination—for the very simple reason that there's no single terrorist hierarchy that would even be able to agree on a trigger. Further, different factions of terrorists will claim credit, and results, and causes, to advance their own factional agendas.
One would think that reading closed minds is easy; it's not. So I make no claims to understand the terrorist motivations, what they hoped to accomplish, their target choices (which appear to have been dictated as much by existing security precautions as anything else), or anything else. Anybody else who does—even terrorist mouthpieces—is either lying about it or just doesn't know.
Final note: It looks to me, from the BBC's helpful map, like three out of four bombs missed their probable targets by a matter of a couple minutes, which is no comfort to the dead or their families or anyone else involved; it is only to say that it could have been much, much worse if four (instead of just one) of the devices had been at the nearby major junctions.