Hi Dan!
I agree with Karen. I know an elderly couple who are both quite observant--and, as far as I know, both atheists. (At least the husband is--not sure about the wife.) That's a bit unusual, but it's not a contradiction. You can be an atheist or an agnostic and still be a fully observant Jew.
Judaism is much more about what you do than what you believe. There's no checklist of beliefs you have to agree on and no articles of faith. Hence we have no creed--even Maimonides couldn't quite pull off a statement of beliefs for all Jews; although he tried with his 13 Principles.
Sometimes I sit in Torah Study, quietly smiling at the vast array of viewpoints people espouse. One guy can hardly stand God and blames God for all the trouble in the world, yet there he is week after week praying in synagogue. Another guy has a very God-is-all-goodness-and-light view. One woman is a practicing Buddhist--she sees God in everything and everything in God.
And different branches of Judaism quarrel more about practice than beliefs. We can all agree on things like loving our neighbors and the importance of the Torah. We don't all agree on Shabbat observance, kashrut, egalitarianism, gay marriage, etc.
But nobody's fighting over doctrine. The only quasi-doctrinal issue I can think of between branches is how much weight to give the Talmud--and, since the Talmud is so tied to observance, even that issue is more about practice than beliefs. There's really not much doctrine to fight over.
(I should mention, too, that even your practices don't determine what synagogue you go to. Lot's of Jews who aren't Orthodox in practice nonetheless belong to Orthodox synagogues. And some members of my Conservative synagogue are quite Orthodox in practice, but quite radical in their beliefs. And, just as there are gay Catholics who won't become Episcopalians, there are gay Orthodox Jews who won't become Conservative or Reform. .)
There's also no one right way to read Scripture--Jewish tradition tends to preserve majority and minority interpretations and everything in between. In a Torah Study in any synagogue you'll hear all sorts of arguments over interpretations, textural criticism, etc.
Since ethics and observance are what's important in Judaism, rather than any articles of belief, it's hard to shock fellow members of a synagogue or even annoy them with your beliefs--or lack of them. (It is possible to get a good argument going, though--it's quite easy, actually.) I suppose the most annoying thing you could do is become a Christian. (There's still a fear of assimilation in Judaism and a distaste for the efforts of some Christians to convert everyone.) But even that won't get you kicked out of a synagogue.
(I found this out from family members and friends who, largely due to intermarriage, have been caught between the two faiths. And I hasten to add that there's never any issue with the Christian spouse or children of a Jew attending synagogue--or any gentiles attending synagogue, for that matter. The problems arise when one person tries to combine both faiths.)
But this is all just a long-winded way of saying it's perfectly possible to be agnostic--or even atheist--and still be an observant Jew. And you'll find the occassional agnostic or atheist in Orthodox, Conservative, Reform and Reconstructionist shuls.
~Rose
