Last year when Apple discontinued MobileMe service, and migrated accounts to iCloud, they gave all previous MobileMe users a 20GB complimentary storage upgrade for a year.
This free upgrade is ending on September 30th, so iCloud users that were migrated from MobileMe will need to pay $40/year to keep their 20GB plan. Apple is sending out emails to former MobileMe users informing them of this upcoming change.
If a former MobileMe user does not sign up for a 20GB plan, their storage space will be downgraded to 5GB. If the user has more than 5GB stored in iCloud when this happens, the majority of their iCloud services will temporarily stop working, including: iCloud Backup, iCloud Mail, and Documents in the Cloud.