The plastic explosive "Goma 2" used in the bombings has not been used by ETA for several years. They use "Titadyne" now.
It's not like ETA has any ideological commitment to any brand of explosives, you know. They'll use any explosive they can lay their hands on as long as they know how to handle it. For some time they've being using Titadyne dynamite they were able to steal in France, but if they have found some new convenient source of Goma 2 it wouldn't be surprising in the least they went back to it. By the way, Goma 2 is manufactured and commercially available in Spain, while Titadyne is not, as far as I know.
The detonator was made of copper; ETA prefers ones made of aluminium.
See above. It's more a matter of what they can get hold of than of "preference".
The stolen van used to transport the bombs contained one audio tape with koran verses.
A tape that does not include any controversial content and widely available at stores. The only "hard evidence" pointing to Islamic terrorists and very feeble evidence at that.
Fingerprints found in the van do not match with any known or suspected ETA member.
Of course for such an important operation ETA would likely try to use members who aren't known to the police. Many of their previous terrorist operations have been frustrated because the police recognized somebody involved.
For the first time in its history, ETA has condemned the attacks. They have never done this before.
ETA has not condemned the attacks. They have denied responsibility for them, as they did for their previous deadliest attack, the Hipercor massacre where they killed 21. Some political groups and media usually close to ETA have condemned the attacks, though, but it doesn't necessarily follow that ETA does too.
Islamic fundamentalist have threatened spain with retaliation for its involvement in Iraq.
As they have threatened many other western countries at one time or another.
At the time before the bombings, ETA members handed out leaflets calling for sabotaging the spanish rail system.
They were more probably mere ETA supporters, and they're continuously calling for sabotaging one thing or another, so it's not a very big deal.
In contrast to most al-Qaida attacks, this one was no suicide attack (or so is believed at this point of time).
I'm no expert in al-Qaeda but I believe most of their attacks do not involve suicide bombers.
Regarding the confession letter in the London newspaper, their is doubt in its authenticity. al-Quaida has never been so quick to claim responsibility.
There's more than doubt, it's widely disregarded as bogus. It seems the same group making the call is known to make false claims of responsibility, such as for the blackouts in the U.S.
All in all, I'd say the evidence points strongly to a Spain-based terrorist group, whether it's ETA or, say, a new Spanish al-Qaeda cell. But we know ETA exists while we don't know about the latter. So right now ETA is probably the safest bet.