Update: I asked PayPal for details, and was told that it can currently be used by merchants in the U.S. selling digital content to customers in the U.S., when PayPal is the only micropayments solution offered.
(Paraphrased b/c of WebmasterWorld TOS.)
Unfortunately, that disqualifies me for now... :-(
Oh, and BTW, you need to open an *additional* PP account if you want to offer micropayments.