According to a report on Ars Technica, Apple holds the central encryption key for all iCloud data. This means that any data that is stored on iCloud can be unencrypted by Apple of its own volition or at the bequest of legal authorities. Of course, users can encrypt data on their own, which would mean Apple wouldn’t have access to the underlying information. The entire article is worth a read if you use iCloud.
[Source: Ars Technica]