Richard Dawkins assigned theism approximately the same probability.

Well, if we're going to start dropping names, Eliezer would "be substantially more worried about a lottery device with a 1 in 1,000,000,000 chance of destroying the world, than a device which destroyed the world if the Judeo-Christian God existed." It's not the same hypothesis, but it's close, and it's stupid to use ethos so much anyway.

I can understand if you think my confidence in atheism is low, but is so ludicrously low that it deserves 9 downvotes?

No, it's not so low that it deserves 9 downvotes. The fact that it has received so many is disturbing.

What probability would you assign theism?

I think I'll side with the perspective advanced by Eliezer here:

Any numerical founding at all is likely to be better than a vague feeling of uncertainty; humans are terrible statisticians. But pulling a number entirely out of your butt, that is, using a non-numerical procedure to produce a number, is nearly no foundation at all; and in that case you probably are better off sticking with the vague feelings of uncertainty.