Try viewing an app in BlackBerry App World at the moment, and you'll be greeted by a nice Site Maintenance notification:
"Our website is currently undergoing a scheduled maintenance to upgrade our systems in order to better serve you.
We apologize for any inconvenience and thank you for your patience."
We know that with the launch of BlackBerry 10, RIM plans to re-brand the BlackBerry App World name to simply, BlackBerry World. The change was made in order to better suit the new contents that RIM will offer. Music, movies, TV shows, ect. This scheduled maintenance could be part of the transition.
We'll update you when App World is back up and running. I noticed it was first down at about 1:00am EST.