iCloud Storage may report incorrect usage.
You may find that the Storage summary that is shown in:
Settings -> iCloud -> Storage & Backup
is different to the details shown:
Settings -> iCloud -> Storage & Backup -> Manage Storage
For example on my iPhone, iPad, and iMac the summary page shows:
Total Storage: 5GB
Available Storage: 2.6GB
But on the Manage Storage page shows:
Backups 1.1GB
Documents and date: 3.3MB
I just got off the phone with technical support, and the checked my sever settings on the their system and are getting the correct figure of 1.1 GB used 3.9GB available. They suggested that his problem might go away in a future, either by apple changing something on the server or by releasing an iOS update.
It took me an hour on the phone to get this information so i wanted to spare anyone from that fate.
Hope it Helps
Alexie
iPhone 4, iOS 5