Does Apple scan iCloud e-mail like Google does with Gmail?
With MobileMe, Apple did filter (and block) email:
http://www.cultofmac.com/103703/apple-may-be-invisibly-filtering-your-outgoing-mobileme-email-exclusive/
However, I tried the test mentioned in the article using iCloud email, and the email went through just fine.

EDIT:
Turns out they do, just not in the same way.
http://www.cultofmac.com/217557/apples-deleting-icloud-emails-that-contain-the-phrase-barely-legal-teens/