Specifically, some players seem to be finding that it doesn't take any skill to wind up hacking someone's account accidentally. And all it takes is a few log-in attempts to find yourself with access to someone's account name, password, and billing information for all of a player's NCsoft games.
Aion fans first began reporting that they were finding themselves getting accidentally logged in to the master accounts of other players. It didn't take long to put two and two together -- after all, an unscrupulous player or two could easily exploit this to drain Guild Wars accounts dry without ever being detectable. Poster Erys Vasburg on the Guild Wars Guru forum wrote a lengthy post detailing many of the pieces of evidence suggesting that the source of the much-feared hacks have been just that, despite repeated statements from Support Liason Gaile Gray to the contrary.
Community Manager Regina Buenaobra has officially posted a response that re-states the position that ArenaNet has held since the beginning: the account hacks are unrelated to the NCsoft master account and the recent events are simply a coincidence. Oddly, however, she also mentions that changes have been made directly to the NCsoft master account page and password change procedure in response to their escalation of the issue, which raises further questions about how much truth there might be to the rumor. We've e-mailed a few questions to ArenaNet in hopes of getting a more definitive response, but thus far none has been received.
Until everything is resolved, many posters are suggesting that you store any and all valuable items in a second Guild Wars account not linked to an NCsoft account. Kill Ten Rats has an excellent post consolidating a number of the threads and discussions into one place, if you're looking for a bit more info. While this is still a rumor, it's being reported from multiple sources, which makes it a bit more questionable, and we can only hope that the truth of the matter becomes evident sooner rather than later.
[Thanks to Daniel for the tip!]