iCloud on Mountain Lion: one account per Mac or one per user?
That apple ID applies only to the user account that is created during the setup process. If you look in System Preferences, you'll notice the following option called "Apple ID", under a test account I created:



Additionally, when logging into the new test account, I was presented with the iCloud login screen which you saw during initial setup of the OS install on your Mac. 

If you restored your iMac from a backup, or installed Mountain Lion over Lion, 
you will already have access to those accounts without seeing the iCloud setup screen. In that case, open System Preferences, click on Users &amp; Groups, and set up your iCloud account as you wish.

So, one iCloud account per local account. This would appear to be the case on Lion as well.