The World of Warcraft Forums have an update on issues users have been encountering related to instance capacity (thanks Ant and WoW Vault). Word is:

As mentioned in previous updates, we have been working diligently to address recent instance capacity issues through a number of reconfigurations and optimizations. These modifications have been an ongoing effort, and we’re pleased to announce that the process is now complete on all North American realms. Since the optimizations, all realms have shown an improvement in instance performance as well as a noticeable reduction in the amount of players hitting the instance limit. Please be assured that we will continue to monitor the situation closely and will make further changes as necessary. As always, thank you for your patience and understanding regarding this issue.