I think they're slowly nudging people to switch, .mac & MobileMe are going away about June/July, may as well set it up now.
I understand .mac mail will still come through. Do not delete the old account yet.
You cannot use .mac or MobileMe as type of Account, you have to choose IMAP when setting up, otherwise Mail is hard coded to change imap.mail.me.com to mail.me.com & smtp.mail.me.com to smtp.me.com, no matter what you try to enter.
iCloud Mail setup, do not choose .mac or MobileMe as type, but choose IMAP...
IMAP (Incoming Mail Server) information:
•Server name: imap.mail.me.com
•SSL Required: Yes
•Port: 993
•Username: example@me.com (use your @me.com address from your iCloud account)
•Password: Your iCloud password
SMTP (outgoing mail server) information:
•Server name: smtp.mail.me.com
•SSL Required: Yes
•Port: 587
•SMTP Authentication Required: Yes
•Username: example@me.com (use your @me.com address from your iCloud account)
•Password: Your iCloud password