Here are some figures: http://www.lhup.edu/~dsimanek/sci_relig.htm
But, it's a tricky question, and this is why.
Being religious requires having beliefs which are not based on valid evidence - being a scientist requires abandoning all belief altogether, and forming opinions that are based on valid evidence.
So, the scientific mind set is fundamentally incompatible with the religious mindset.
Science does not require one to believe or not to believe in God/Gods - it only requires that, if one forms an opinion about the topic, one bases that opinion on evidence, and make the evidence public, ready to be scrutinized and put to the test of time.
Thus, it's not surprising that great many scientists are atheist.
Note that being an atheist simply means not believing in a deity or deities. Being a scientist requires you to go a step beyond that - science recognizes that noting is 100% certain, so, a scientist cannot truly be a gnostic atheist.
One can, however, make claims about the existence of a specific kind of God (the Christian God, Allah, Zeus...), and form an evidence-based opinion.
This is why any scientist who claims to be a believer is essentially living by double standards.
In that sense, 100% of scientists are atheist. Some small X% of them don't seem to know it. :D
EDIT: I've been asked to expand on this, so here.
However, note that my point was that such figures don't really answer your question - the question is fundamentally ill-posed. None who values the principles of modern scientific thought (form models based on available valid evidence) can honestly claim that is simultaneously religious in the same sense religious people are (who form models of reality based on invalid "evidence", essentially hearsay).
It's what being a scientist is by definition. (science, scientist)
The article I linked to shows some figures, for 1914, 1933, 1998:
BELIEF IN PERSONAL GOD 1914 1933 1998
Personal belief 27.7 15 7.0
Personal disbelief 52.7 68 72.2
Doubt or agnosticism 20.9 17 20.8
BELIEF IN IMMORTALITY 1914 1933 1998
Personal belief 35.2 18 7.9
Personal disbelief 25.4 53 76.7
Doubt or agnosticism 43.7 29 23.3
Note: The 1998 immortality figures add up to more than 100%. The
misprint is in the original. The 76.7% is likely too high.
Note high percentages for non-belief. But, how was the study done?
Larson and Witham present the results of a replication of 1913 and 1933 surveys by James H. Leuba. In those surveys, Leuba mailed a questionnaire to leading scientists asking about their belief in "a God in intellectual and affective communication with humankind" and in "personal immortality". Larson and Witham used the same wording [as in the Leuba studies], and sent their questionnaire to 517 members of the [U.S.] National Academy of Sciences from the biological and physical sciences (the latter including mathematicians, physicists and astronomers). The return rate was slightly over 50%.
Ask yourself this: are this questions good? The word "belief" is problematic - as it doesn't have the same meaning for religious and opinion-based belief. So, what does a scientist mean when by "personal belief" or "personal disbelief", exactly?
Are they talking about the concept of God (creator, ultimate cause) in general, or is it a deity with more specific characteristics? Is there doubt in disbelief?
My point is, studies like this don't paint a representative picture.
If the right questions were asked, the percentage would be much lower (now that's an unsupported claim - but different at least).
If a scientist would provide evidence for God, and this evidence was confirmed, and most scientists started "believing" that there in fact is a God, this wouldn't classify them as theists - as their "belief" would actually be an opinion based on factual evidence.
And yes, they could be wrong. But science doesn't forbid you to be wrong - it only asks of you to admit it when you are presented with contradicting evidence. Religion, however, does - if you doubt, you're doomed.
And that is just not compatible with science.
Those few who have religious beliefs would never apply such reasoning to scientific research, and if they did, they would be laughed at, as that is not science, and they wouldn't be able to defend their claims.
So such people have double standards.
Another thing: creationists and the like are not scientists - not because of their claims, but because of invalid (even fabricated) evidence, and because of flawed logic they use to support them.
I know many of you won't like this answer - but there you have it, and it's all yours to judge.
OK. Maybe this answer is not quite adequate, but that's because the question, as posed, doesn't really make sense - at least not without first precisely defining terms like "belief", "atheist", and even "scientist".