Danny Sullivan has posted a few videos from Google’s Zeitgeist event, and one of them is a fairly entertaining banter between Sullivan and Google CEO Eric Schmidt.  Sullivan  asks why Google doesn’t list its 200 ranking factors. Schmidt’s response:

"Because we change them. What would happen is, you’ve asked me this question for the eight years I’ve worked with you, so it’s the same question. Why don’t we publish these things. And the fundamental answer is we’re always changing. We’re always changing, and if we started saying here’s how the black box works, then all of a sudden huge incumbencies would come out about this change and that change, and we just don’t want that pressure."

He also says it’s a "business secret".

"I agree, Google’s exact ranking formula is a business secret," writes Sullivan at Search Engine Land. "But I wasn’t asking for the secret recipe, exactly how all the things are mixed to create Google’s special sauce. I was asking what harm there was in listing the 200 various ingredients that are in the sauce."

He provides more perspective here. 

We do know one thing. People obsess over PageRank too much. There are 200 factors.