At American Prospect, Harvard terrorism expert Jessica Stern looks at the possible connection between Basque separatists and Al Qaeda:

I have to wonder whether there's cooperation between ETA and Al Qaeda, and what this relationship might consist of. Al Qaeda is pragmatic and likes to avail itself of local operatives, expertise, and languages. They especially like to recruit locals. Al Qaeda has a large presence in Spain, so looking for partners like ETA would be at the top of their list. We know that the majority of people in Spain oppose the war in Iraq, so it makes me wonder whether some members of ETA have been infiltrated by the Al Qaeda network. There's also the chance that Al Qaeda might be recruiting within ETA.

As to the credibility of the Al Quaeda group claiming responsibility for the Spain attacks, Stern says:

The group claiming to speak for Al Qaeda is notoriously unreliable -- they even claimed responsibility for last summer's blackout. Intelligence officials really don't know anything about the group.