Dirty money is a quaint concept because it's hard to imagine any other kind. Money is literally dirty -- some 18% of coins harbor pathogens, according to a 1998 study by Shirley Lowe, an assistant professor at the University of California -- and many great fortunes were made out of less-than-honorable work.

Moreover, in today's global economy, the path from consumer to product to manufacturer to raw materials is so labyrinthine that someone somewhere along the way is sure to have been exploited or to have gotten a raw deal. Welcome to capitalism and fasten your seat belt.

At the same time, questionable profits are hard to ignore when Google is involved, mainly because of Google's informal corporate motto, "Don't be evil."

So here's the issue: some portion of Google's revenue comes from search keywords associated with questionable and illegal activities.

A post on the F-Secure blog yesterday highlights one example of this. "[T]ry searching for 'bulletproof hosting,' and you'll get a bunch of Google sponsored links for companies that sell hosting for spammers," observes Mikko Hyppönen, F-Secure's chief research officer.

Bulletproof hosting isn't illegal per se, but it's highly dubious.

Accepting ad dollars from spam-friendly ISPs is not just Google's problem. Ask, Microsoft, and Yahoo all sell "bulletproof hosting" as a search term. Maybe search engines see such money as more dusty than dirty. Spin it a bit, the dust comes off, and it's good as new.

Google does steer clear of some controversial keywords. Search for "tobacco" and you won't see any ads. That's a stance many print publishers refuse to take. Try "guns" and you won't see ads on Google or Yahoo, though you will if you use Microsoft's Live.com.

But then there are keywords like "oxycontin." Amid ads for detox services, Google also serves ads offering "Get Pain Pills Easy." So no tobacco ads, but controlled opiates are OK.

Google has clearly thought long and hard about permissible AdWords content. Its rules are extensive and commendable. In fact, Google explicitly states, "Advertising of bulk marketing products is not permitted if the stated or implied use of the following products is unsolicited spam."

Of course, having a rule is easier than enforcing it.