There are people (individuals, and entire enterprises) dedicated to financial shenanigans of the black hat variety. If Paypal didn't have a robust risk prevention system, they'd have been annihilated a long time ago.
In fact, I often spot fraudulent charges very quickly and have to email my third part processor to refund or cancel the payments.
I was pointed to http://maxmind.com by another HN poster, which is an API that'll give you a probability of a charge being fraudulent so you can verify it before passing it to the payment provider. I'm going to test that out in combination with Stripe.
I'd definitely be interested in seeing a write-up about integrating Stripe with third-party fraud detection systems.