According to a report over at Phoronix, the information was revealed during the Free Open-Source Developers' European Meeting held the previous weekend at Brussels in Belgium. It was also revealed that this will be a fully native Linux port of the application, and not dependent on Wine or similar compatibility layers that let Windows applications run on Linux.
From a source in Brussels, Belgium during the Free Open-Source Developers' European Meeting (FOSDEM) this past weekend, I was informed that Microsoft is having a "meaningful look" at a full Linux port of Office thanks to Linux showing signs of commercial viability on the desktop. Right now some versions of Microsoft Office will work under Linux via the use of Wine or CodeWeavers' CrossOver to varying extents, but this port being evaluated internally at Microsoft is a fully native implementation. Evidently there's already some port to unknown completion that has been done internally at the company.
As the report notes, Microsoft has already confirmed its plans to release Office for iOS and Android, and with the latter being a Linux-based operating system, porting Office to Linux seems the next logical step.
It'll be interesting to see if Microsoft expects to be rewarded for its work in bringing Office to Linux in terms of being paid for licenses as on other platforms, or if it just chooses to give the software away in true Linux tradition. As the report further notes, Microsoft has probably observed the willingness of people to pay for paid services around the Linux platform, and hopes enterprise users will be happy to shell out bucks for Office.
Detractors of the move may point to the fact that Office for Linux is one less reason for people to buy Microsoft's Windows, but Microsoft has developed multiple versions of Office for other platforms like Mac. Of course, Office for Mac has a history of its own going back to the time Microsoft made an investment in Apple. Part of the terms of the deal was Microsoft's continued support towards developing Office for Mac, so a strict comparison between Office for Mac and Office for Linux probably isn't fair.