It's been awhile since I talked about this topic, but Juan Cole on Informed Comment has a post about an MI6 memo from 2002 that was leaked recently.

Juan Cole writes...
It summarizes a report to Blair and others in the British government by Sir Brian Dearlove (This is the press release when he was appointed in 1999). The head of MI6, or the foreign intelligence service of the UK, is known as "C."

Here is the smoking gun:

"C [Dearlove] reported on his recent talks in Washington. There was a perceptible shift in attitude. Military action was now seen as inevitable. Bush wanted to remove Saddam, through military action, justified by the conjunction of terrorism and WMD. But the intelligence and facts were being fixed around the policy.

(emphasis mine)

*sigh* It looks like this isn't finding much traction in our news media. It really makes me sad.