As pre-announced at last week’s The Beat Goes On event in San Francisco, Apple has today turned on custom ringtone functionality in iTunes. The feature enables users to convert certain iTunes Store tracks into ringtones at a cost of $0.99 per ringtone plus the initial cost of the iTunes Store download; other tracks, including ones you have imported from your own CDs, cannot be converted. To support the feature, a new “Ringtones”…