updated 04:45 pm EDT, Thu July 24, 2014
iTunes and App Store difficulties already resolved
[Updated with spreading service issues] Apple online services have been affected by a pair of problems on Thursday, the company's system status page indicates. Still ongoing is a glitch with iCloud Mail, resulting in "slower than normal response" for some users. Trouble appears to have begun shortly after 2PM Eastern.
A more serious problem briefly impacted Apple's digital stores. Between 2PM and 3PM, some people were unable to buy anything from iTunes, the iBookstore, or the Mac and iOS App Stores. As with the iCloud Mail bug, Apple says that "some" users were affected.
Update: As of 4:48PM Eastern, a variety of other services are now also experiencing slowness. These include Documents in the Cloud, Find My iPhone, iCloud sign-in, iCloud Backup, iCloud Calendar, iCloud Notes, iMovie Theater, iWork for iCloud, and Photos. In some cases, iCloud Mail may even be completely inaccessible.
The timing of the issue coincides with the release of the first Public Yosemite (OS X 10.10) beta, which is being hammered with up to a million users attempting to download the update, so the slowdowns for other services could be affected.