Apple is sending out warning letters to iCloud customers that received a complimentary 20GB storage upgrade that the offer expires on September 30th. On October 1st, those users will be bucked down to the 5GB of storage that normally comes with iCloud.
The offer was originally extended to MobileMe paying subscribers who moved their accounts to iCloud between October 12th, 2011 and August 1st, 2012. While iCloud nominally comes with 5GB of free storage, MobileMe customers were automatically upgraded to 25GB total storage.
On September 30th, 2013, those users who have not already purchased additional storage will be dropped down to the same 5GB of free space other iCloud users get. The letter sent by Apple:
Apple's notification Email
If you are using less 5GB and don't intend to store more documents in iCloud, you don't need to do anything. Apple currently charges US$40 per year for an additional 20GB or $50 per year for an additional 50GB of storage.
According to the KnowledgeBase article Apple linked to, "If the storage you're using in iCloud exceeds the total storage available, iCloud Backup, Documents in the Cloud, and iCloud Mail will temporarily stop working. To resume using these features, you can either reduce the amount of iCloud storage that you're using, or purchase a storage plan with a larger amount of storage than you're currently using."