The settings in http://support.apple.com/kb/HT5330 are wrong, heaven knows why: they are the old MobileMe settings which have straggled on a bit and worked for some people but are likely to stop working at any time.
Setting up iCloud mail manually can be tricky in Snow Leopard because its 'wizard' tries to force you into MobileMe. The method for setting it up is described in this illustrated article:
it would seem one of those kbases made sure to mention option-click in the process to allow edits in the IMAP setup.
So, in the 10.6 info:
user name (excluding @me.com)
But, in the iCloud server setting for email clients:
It seems user name under each is different in that one wants @iCloud.com and the other doesn't.
Also, "imap" is added to the icoming and outgoing info.
I set my SL up using the "icloud: mail server settings" instead of the one for 10.6.
But, I was trying to figure WHY there are two different ways and why specifically 10.6 differs.
10.6 doesn't differ. I'm using 10.6 with imap.mail.me.com and smtp.mail.me.com and have been since the beginning of all this, and I have no idea why the first page mentioned is giving the old MobileMe settings. I seem to remember Apple posting a suggestion that if ex-MobileMe subscribers did nothing their mail would just continue to work - which would have been perfectly logical - but the experience seems to have been that it worked for a time and then stopped working. The servers appear to be different for different goegraphical areas (so that one server doesn't have to cope with the entire planet) and presumably different areas are losing the old settings at different times.