This is something I keep hearing from certain theists who feel the need to argue with atheists.  It came up during Christopher Hitchens' recent NPR interview (21 Mb .mp3 file).  I've heard it from the Kirk Cameron crowd.  I've heard it all over the place.  Let me summarize it:

There's so much in our world which is unexplainable by science, how can you not believe there is a God who created these things?

I hate this argument, mostly because it makes a number of logical errors.  Christopher Hitchens would go further to say that it is ridiculously arrogant.  During his NPR interview when a lady called in and put this to him, his response was (paraphasing):

She seems to imagine what isn't explained can be explained by her.  I wouldn't presume any such arrogance.  What have we been wasting our time for?  With all these inquiries into nature and the natural order--there's a lady in Virginia Beach who knows all about it, and she even knows who's responsible.  We should have asked her.  Isn't it amazing how religious people claim such humility and yet make the most fantastically arrogant claims?.

I wouldn't take Hitchens' stance that the caller was arrogant, only that she simply doesn't understand the logical problems with her argument.

First, why this peculiar jump from "unexplained" to "unexplainable"?  There are questions for which science has no answer other than "I don't know", but some people don't seem to realize that means "I don't know yet."  Surely there are some questions science cannot answer right now because nobody is investigating that particular question, or the answer requires evidence that hasn't been found yet.  But it doesn't follow that these mysteries are never going to be explained by science... therefore it's quite silly to refer to them as "unexplainable".  There is a big difference between unexplained and unexplainable.  A good analogy would be whether or not you have visited Easter Island.  If not, from your perspective, Easter Island is unvisited.  Is it therefore true that it is unvisitable by you?

Secondly, why the immediate leap to God?  So there is a question we don't know the answer to, why must the answer to that question be God then?  You do realize at some point the question will be answered and in all likelihood God will not be the answer.  What then?  If God is not the answer then, why should he be the answer now?  Consider that 1000 years ago, we didn't know what caused lightning bolts to rain from the sky, and many people therefore said it was the work of one god or another.  Now we know exactly why lightning happens, and God doesn't appear in the recipe.  Why should today's questions unfold any differently than the questions of 1000 years ago?

Finally, and this kind of bugs me the most, when will we know everything there is to be  known?  When will we have visited every single planet in the entire universe and categorized everything on them?  In all likelihood?  Never.  Every new scientific discovery always brings new questions.  Always.  If the universe is infinite, then so is knowledge.  Because of this there will always be unanswered questions.  Which makes the existance of such questions rather mundane.  It's simply a natural side effect of learning new stuff that new questions crop up.  It's not an amazing, and compelling, and mysterious thing that there are unanswered questions.  If you are looking for the empty spaces on the shelves of knowledge for a place to put God, you are going to be moving him around forever.  Those spaces are reserved for books that haven't been written yet.  If you want to stop having to reinvent God every time science fills a gap, then maybe you should put him where he belongs... in the mythology section.