The Bin Laden tape had the following paragraph in it. In retrospect it is clear that he activated the attack - at the time, buried in 46 minutes of talk, it was difficult to recognize. There is at least 1 other activation message in that tape, probably 2, that I have been able to identify.
In the tape he talks about the Spanish ruler being held accountable. At one glance the message appears complimentary; the doublespeak doesn't come through in the translation very well; it's more apparant in the original Arabic. It is quite clear that he plans to hold the Spanish accountable for their actions. The Madrid attack was retaliation against Spain for its part in the invasion of Iraq.
Take a look at this excerpt from Bin Laden's tape of late December/early January that was only partially released at the time, but was released in its entirety last week:
Thus, the situation of all Arab countries suffers from great deterioration in all walks of life, in religious and worldly matters. It is enough to know that the economy of all Arab countries is weaker than the economy of one country that had once been part of our [Islamic] world when we used to truly adhere to Islam. That country is the lost Andalusia. Spain is an infidel country, but its economy is stronger than our economy because the ruler there is accountable. In our countries, there is no accountability or punishment but there is only obedience to the rulers and prayers of long life for them.