Some attacks have probably been broken up, but in the end we're playing defense when preventing terrorist attacks. They only need to be good or lucky once, while we have to be perfect.
Overall I think al Qaeda is being smacked around pretty hard. Luckily they're not very bright. They picked a fight with the Saudi government, and as a result they've lost a good part of their infrastructure there. They've also been bomb-happy in ways that alienate the muslim base, as in the car bombings in Karbala or Turkey.
The problem is that they don't need a huge, well-oiled organization to pull off some attacks. The best we can hope for right now is to prevent them from being organized enough to pull of something really horrific, like a bio attack or a nuke. But we'll have to live with the possiblity of car bombs for decades. Any idjit can pull that off, as McVeigh demonstrated.
My guess is that they'll do _something_ before the election. They seem to be trying for a spectacular, but if they can't do that they'll probably settle for something smaller, just to prove they're still relevant.