While the application is missing for some users on iCloud.com, navigating directly to the Photos app URL also results in an error. We were able to have the application successfully load; however, it was very slow and eventually threw us an error.
The iCloud Photo Library was initially slated to launch alongside iOS 8, but Apple delayed its rollout until iOS 8.1
Apple has yet to update its System Status page to reflect any outages, but we'll keep you updated on when the photos app returns!
Update: The service seems to be restored for some users