Might I interject that the poster who claims to be the 'defendant' doesn't have to be him, I can easily use a server (legally) from Warschau to post comments on ENworld, that means that pretty much any idiot could do it ;-) There are individuals on the internet that like to make posts that make 'waves' in the community, very recently ENworld has suffered from such a 'prank'. For the initial post to be considered evidence, it needs to be connected to the actual 'defendant', that isn't going to happen unless ENworld is going to provide the IP address to WotC/Hasbro. I do believe that the ENworld servers are located in the US, so some sort of legal demand could be made (either to the owners of ENworld or the hosters of ENworld) for the posters IP address. RPGnow/DTRPG probably already has the 'defendant's' IP address when he bought the products. He currently might have another, a Polish ISP might need to be contacted for that, I don't know how easily Polish ISPs give out their clients IP addresses...
For all we know this is the actual 'defendant', it might be a kid looking for some attention, it might even be someone affiliated with WotC/Hasbro or it's legal team (I'm not making some claim here, it's just not an impossibility). I would like to caution against making more of this, then that it actually is.
I would also like to point out that while the 'defendant' claims to have lent the pdfs to friends (which might or might not be legal under Polish laws), those might not have shared the files on the internet. Hacking is a favorite pastime of a certain subculture, so the files might have been 'stolen' before being shared on the internet. Another option might be that the suppliers where hacked and an illegal copy was obtained that way (RPGnow was hacked in 2007 and customer information was obtained). I know from experience that I had access to certain DTRPG pdfs before they where released (although I did pay for them before downloading, DTRPG has fixed that issue), so it's not beyond the possibility of reason copies where obtained from the suppliers. I'm curious if WotC can make the direct connection that the actual files the 'defendant' downloaded are the same as the ones being spread. As far as I know, RPGNOW/DTRPG generate the file once when you buy it and then stores it on the servers to be downloaded again. An external party could access the files and bypass the log function. Not only the 'defendant' had access to the files, also the employees of the supplier, the suppliers hoster, and the backup service they might use. Then we have the possibility of the watermark being manipulated by an evil minded third party, data can be manipulated...