Your iCloud address can be your ID under only two circumstances:
1. You had a DotMac/MobileMe account which was converted to an iCloud account when MobileMe closed: your MobileMe address was the login and continues to be the ID in @me.com and @icloud.com versions.
2. You can choose an iCloud address and use it as the ID when you first create the account if you do so from the iCloud Preference Pane, as detailed here:
Re: I wish to transfer my Apple ID to a new iCloud account
However once you have created an ID using a non-Apple address you can change it to any other non-Apple address which hasn't previously been associated with an Apple ID, but you cannot make your @icloud.com address your ID (when the account was created is irrelevant).
In any case there are points against doing this: it's better if the login is not the same as the address which people know, as a security precaution. Also if you are in a situation where the only address Apple have for you is the iCloud address, they cannot contact you if you cannot access the account - for example if you forget your password.
