Hum, I think it greatly depends of the country. In some countries (like Germany) you officially declare your religion because it affects your taxes. In some others like France, the state just doesn't care about your religion, but (both public and private) institute perform samplings in which they ask people if they have a religion and if what it is.

On the polls, results depend greatly on how the question is formulated (depending on the wording, people who identify themselves as culturally christian but don't believe in god will answer differently), but the latest detailed poll done, according to Wikipedia, gave something like (simplified and translated by myself) :

Sondage CSA 2006-2007

Catholics : 51 % among which :
Believing catholics : 27 %
Agnostics catholics : (don't know if God exists) : 15 %
Atheist catholics (don't think God exists, but identifying themselves as culturally catholics) : 9 % Atheist : 31 % Muslims : 4 %
Protestants : 3 %
Jew : 1 %
Other/not answering : 10 %

Which leads to a majority (55%) of atheists or agnostics. But if just asking '"are you christian ?" you'll get a majority of christians... the joy of polls and statistics.