Microsoft acquires Office.com domain
Up until recently, the domain belonged to ContactOffice. It looks like ContactOffice began transferring its service to a new web site in June, so the acquisition has probably been in the works for a little while, but the transfer officially went through on August 4th.
Single-word domain names like Office.com don't come cheap. But the move was probably a smart one on Microsoft's part. The company's current homepage for MS Office is office.microsoft.com, which isn't quite as easy to remember.