It gets interesting when you look at Pottermore's trademark page, which lists the trademark as being owned by Warner Bros. Entertainment Inc. and describes the product as follows:
Providing multiple-user access to a global computer information network; providing on-line chat rooms and electronic bulletin boards for transmission of messages among users in the field of general interest; providing on-line communications links which transfer the web site user to other local and global web pages; providing on-line facilities for real-time interaction with other computer users concerning topics of general interest.The Guardian notes that Pottermore could stand for "Potter Multiplayer Online Role-playing Experience," among other things. An editor at HPANA was one of the few to get a glimpse of the full site, commenting, "It is breathtaking in scope, detail and sheer beauty." Meanwhile, on YouTube's JKRowlingAnnounces channel, a timer is counting down to an announcement as to why "the owls are gathering" sometime on the 23rd.
Of course, a Harry Potter MMO has been heavily rumored for some time now, perhaps in association with Turbine (which is owned by WB). It's a tenuous connection but a possibility as well, so we'll be keeping our eyes on the site to see what transpires!
[Thanks to reader Alen for the tip!]