I'll admit that the September 11th attacks were pretty impressive. They killed Walter Weaver and about 3000 other people, demolished a major landmark, and put a serious dent in the US economy. The US returned the favor by knocking over two regimes. My understanding of the matter is that since what the terrorists allegedly wanted was for US troups to get pulled out of the middle east, that last development was not what they were looking for.
That brings us to this latest attack. I'm guessing that part of the motive was that the terrorists wanted those UK troups our of Iraq. I can't think of what else it might be; if they wanted to make the UK look bad the best thing would have been to do nothing and let the new services cover the G8 protesters. On the other hand they've pretty much just guarenteed theat the UK will stay in Iraq. Having troups there was pretty unpopular with the general public when it was seen as going along with Bush, but there's nothing that will get the Brits committed to a war like getting bombed by the other side. When I was a little girl visiting my Dad's hometown of Hull I saw the some of the sites that had been bombed during WWII, so I'm pretty clear on this point.
So, the bad guys have now succeeded in killing a few dozen more people, damaging the underground and blowing up a bus. Other than that it hasn't gotten them much. The economic damage to London is likely to be minimal and short-lived. Heck, the underground needs some renovating anyway. Given the likely response I don't think the terrorists are going to like the net results of their latest actions.