Blizzard has added two advertising bars to their official forum pages, one on the top and one banner along the side. And it's got players concerned -- there's a large forum thread growing even bigger by the minute right now. The main complaints seem to revolve around a few issues: the design breaks the forum layout for some users, the ads are possibly a security risk (they aren't hosted by Blizzard -- more on that in a second), and of course the issue that we're paying every month to be able to look at ads on the forums.
As for the design argument, everything is a matter of taste in that way -- if Blizzard doesn't feel the ads break their forums, then we probably won't see a change. They didn't, for example, put ads in between the forum posts, and there are no ads on the official front page (save for Blizzard's usual advertising of World of Warcraft itself). As for the second issue, that's a stickier one. The ads are served by a company called Atlas Solutions, which appears to be a division of Microsoft, which, you'll remember, is the same company that Blizzard recently made a deal with for ads on their Battle.net service. Right now, the ads are mostly for extended universe items from Blizzard partners like Upper Deck, but players are worried that an unsavory advertiser could get in there with a keylogger, and then suddenly half the forums are hacked.
That seems unlikely -- Blizzard will definitely have a strong say over what does and doesn't go up in those ads (no gold advertisers, for example), so as much as the Flash-based ads are a security risk, Blizzard certainly must think they're well protected against any issue like that. And finally, the question of whether it's right to show a paying audience ads is up to you -- Blizzard seems more than happy to charge for optional experiences in their game, and going on the forums certainly isn't a part of the game proper. They likely believe that while advertising ingame would be wrong, advertising in an "optional" place like the forums is consistent with their moneymaking policies so far. But the fans seem very against it -- we'll see what choices Blizzard decides to make from here.