A quick note from WotC:
The 10:1 ratio that Greg references is for PDFs only – it has nothing to do with the physical books. For every one PDF purchased legally, there were at least 10 downloaded illegally. And yes, we can track it.
I still say BS.
The only reason they had firm numbers of downloads with PHB2 was because some idiots uploaded it to Scribd, and Scribd provides information on numbers downloaded. Comparing those numbers to actual numbers of pdf's sold gives a ratio. If several thousand were downloaded from Scribd and several hundred sold, you get a ratio of 90%. However, I doubt you can track torrents, Limewire type services, and just plain old emailing or swapping on usb drives.
After this mess, I doubt anyone would be foolish enough to put stuff on Scribd again. But if they did, I would imagine it would be harder to catch them, since the sale of the pdf had the hidden watermark. A good old fashioned scan of a book has no such identifying features.
I know next to noting about how to pirate stuff, but it seems to me that if you are left only with other means of distribution than Scribd, which while they may be trackable on som individual basis if you know the person doing it and get a warrant to track all traffic through his IP address, I doubt you can just go out to some master internet tracker and get numbers of pirated copies downloaded.
So basically, what WOTC has done here is increase market demand for pdf's, taken away some of the guilt of doing it as well as their ability to track it, and alienate their customer base.
Nice.