It would be a fabulous move from Microsoft that would help both them and linux to gain even more market share. And no it won't be open sourced but only zealots care about that. The rest want to use office and would gladly to that on linux if it were available.
Photoshop when? ;)
How reliable can this rumor be ? There were no developper from Microsoft at Fosdem as far as I know. At best, if it is true, someone heard it from a friend working at Microsoft. More likely, someone working at Microsoft (but not in the MS Office team) said to a friend that with Valve pushing Linux, they might consider porting MS Office to it if it is a success. And by the time the rumor reached Michael's hears, they had a native port on the way... I don't trust that at all.
Unless they are writing it from scratch with a whole new team... No. Ain't happening.
We have a number of people, myself included, who run Linux on their desktops (although not officially sanctioned by IT) since we develop for Linux. So we generally have to run a VM with Windows for Office, especially Outlook. That's a hassle I could live without.