Posted today on the official website;
In our effort to try to continually improve everyone's game experience, we recently updated some of our database hardware. This update required extended downtime for certain realms as we transferred some of our data from our launch servers to our new servers. The hardware update and data transfer was intended to split the load among multiple servers, which would in turn improve performance across the servers involved.
Even though our updated hardware represents some of the best technology currently available, configuration of this hardware proved problematic in some cases. We're working through the challenges diligently, and we feel that resolving the issues that some players are experiencing is just a matter of time now.
We do appreciate your patience and enthusiasm about playing World of Warcraft, and we regret that you faced periods of high traffic this past weekend that caused some of you to have a poor game experience. So, due to the extended nature of the server issues, we will be providing a free 48-hour extension of play time. As with previous extensions, this will show up as free extra time on your current billing period (or on your trial period, for those of you who purchased the game recently). We hope you'll bear with us as we continue to work hard on implementing changes for long-term stability.
We've received many calls and emails from players telling us how much they're enjoying World of Warcraft and that they would just like to have a more consistent connection experience. In order to achieve this, we will continue our analysis, and we'll continue to take further measures as needed, such as occasional login queues. However, we will also continue putting every effort into consistently delivering more information about related technical issues. Again, we appreciate your ongoing patience; our ultimate goal is to provide the high-quality gaming experience that you expect from Blizzard.