Apple started notifying former MobileMe users on Wednesday that they now have iCloud.com email addresses, although they can continue to use their .Mac and MobileMe addresses, too.
Apple rolls out iCloud.com email addresses for MobileMe users
In a knowledge base article on using iCloud email addresses, Apple stated,
If you are an existing iCloud Mail user with an @mac.com or @me.com email address, Apple has reserved an @icloud.com email address for you. You will receive an @icloud.com equivalent for every active @me.com or @mac.com email address or alias that you have as part of your account.
MobileMe users that migrated to iCloud can watch for an email telling them when their new iCloud email addresses are ready.