Due to recent server issues Tyren made the following announcement on the Blizzard WoW forums.

24 Hour Game Time Extension
Over the past weekend, the realms listed below experienced extended downtimes due to unforeseen hardware configuration issues in light of recent population increases. We understand that such downtimes are not acceptable to our inconvenienced players, and we assure you that such service interruptions are not acceptable to us either. In light of the downtime, we will be providing a free 24-hour extension of play time. 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).

The complete forum post with a list of servers gaining the credit can be found here