According to the Spanish government, the objective of the attackers was to steal passports and documents (presumably for sale on the blackmarket or use by themselves).
ETA is well known for carrying out IRA-style robberies of things ranging from banks and cash sources to construction companies and other sources of explosives and equipment. So I suppose this could be ETA, particularly since the attackers were heard speaking French. But it would be a robbery, rather than the bombings that ETA carries out simply to terrorize and kill. (On the other hand, I suppose the attackers could be Algerians; all terrorists love to get their hands on fresh, unused passports.)
Yes, ETA has carried a lot of robberies in France mainly, but they never have committed them in Switzerland or against an embassy.