Since the start of June I’ve noticed that http://portal.microsoftonline.com redirects to http://portal.office.com.
So far I haven’t seen any official communication about this, so I did a search on Twitter and found that Trey Mayer (@TeryPoint) noticed this towards the end of May as well.
— Trey Mayer (@TreyPoint) May 30, 2014
While the change wasn’t communicated; I appreciate the shorter URL, the redirection seems to work and *.office.com was already listed at Office 365 URLs and IP address ranges – Office.com (and Office 365 URLs and IP address ranges – TechNet).
Valid for Office 365 Wave 15 – 17/06/2014.