Apple has posted a notice on its System Status page that some iTunes Store users "may be unable to make purchases". The company has not provided any further details, nor an estimate for when it will be repaired.
No other Apple services are currently affected, however.
Update: Apple is now reporting that the outages have extended to the App Store, Mac App Store, and iBookstore, as well as iTunes Match, iTunes in the Cloud, and iCloud Backup. The company has yet to list a percentage of users affected by the issues, but based on reports it appears to be fairly substantial.
Update 2: Apple is now reporting that all services are back online as of 10:54 AM Pacific Time.
Update 3 - 11:31AM PDT: Though previously affected services are back on line, Apple is now reporting that some users may be unable to use FaceTime.
Update 4: Apple is now reporting that all services are back online as of 4:00PM PDT.