As per Microsoftâ€™s announcement on the 31st July, App Hub is down today for planned maintenance, and may stay off-line for up to 24 hours.
The downtime is to upgrade the infrastructure underlying App Hub and Marketplace, and Microsoft promises the service on return will result in a more robust submission and certification pipeline for Windows Phone 7.5 apps, and generally improve speed and stability.
Developers have long complained about App Hub being slow, cumbersome and difficult to use, so I am sure they do not mind some inconvenience today for hopefully a much better service tomorrow,
Read the announcement at Microsoft here.
Thanks Mohammed for the tip.