Historical explanations for American religiosity?

I was wondering how people would explain the apparently massive difference in religious attitudes between the US and Western Europe. All the polls seem to indicate that people in the US are far more likely to be religious (or religiously observant) than in Western Europe, and on the face of it this is counter-intuitive. The US has a constitution that studiously separates religion from public life, whereas many European countries have an established church. Here in the UK, all state schools are required by statute to provide religious education and corporate worship, both of which are (I am informed) banned from US public schools. If you were to ask me to predict which of these two societies would be the most religious, I'd naturally plump for the UK - but the exact opposite is true!

There has to be a historical explanation for this difference in attitudes. Any ideas, anyone?

(I'll end by pointing out that I think this is a genuinely interesting historical question which can be addressed by people of all faiths or none, and it'd be a real shame if this thread should descend into a religion-flavoured flame war!!!) 

I guess it's interesting to reflect on what we're considering here. What's more anomolous and in need of explanation - the American religious revival, or Europe's long slide into secularism?