I used to interview people for Google, and "I don't know but here's how I would go about finding out" almost never counted against candidates. What would count against a candidate is if they made a wrong assumption and then blindly proceeded as if that assumption was correct.
I think that's happened before, not for one of my interviews but for one a friend gave. I don't think it counted against them, but they weren't good enough to meet the bar anyway. (That happens a lot - I've never had a non-intern candidate get through, and I've had friends go for 50 interviews without a single candidate getting hired.) We had a good laugh afterwards.
