Maybe this does send a signal to A-Q that they can change the face of politics. Though that conclusion is highly suspect given the anti-war sentiment already present in Spain, and the badly-attempted cover job trying to blame ETA.
I'd say it's more of a harbinger. Most Europeans - having a real historical basis for understanding what war on their soil means - never supported the Iraq war. Those leaders that chose to follow the US lead did so against heavy popular opposition, and the war isn't going according to plan. Not to mention the gravy is all going to no-bid contracts for US companies.
The long-term outcome of this could be to solidify and re-invigorate the European Union in ways never before possible. As elections come and go, the parties that supported the war - and are pro-US - will find themselves running out of friends. Perhaps even the hard-liners who envision the EU as a counterbalance to the US on the world stage get real power.
On this track, popular opinion in the EU can be tilted. From the Europeans I've known - and there are many - I don't think the natural reaction is a knee-jerk "let's kick their ass" response you'd see here in the States (reference quotes above). Given the popular lack of support over many of the US-led initiatives Europe has been involved in, I'd say it pushes things more into a US-EU competition.
That, with the economic indicators, weakness of the dollar, tends to me that W's legacy may be the weakening of the US as a true world power...sure we have a big army that can blow people up, but any essence of diplomacy or statesmanship that previous admins have had is long gone here, and in civilized circles, that's what counts. Let's hope he doesn't invade Spain next.
-doug