Everything they've wanted.

Atrios:

The conservative cult's mass death wish is obviously based on a faulty premise, that if there's a terrorist attack they and Dear Leader will somehow be vindicated. Of course the reverse is true. When it comes to "the war on terra," George Bush and the conservative movement have pretty much gotten everything they've wanted. Democrats and dirty fucking hippie bloggers, despite complaints, haven't managed to stop the Bush administration from doing what they think is important.

So if a massive terrorist attack happened, it wouldn't be a vindication of what they've been doing, it would be proof that they failed to do what George Bush claims is his most important job.