And evn if NBC's "Saturday NIght Live" producer Lorne Michaels winds up producing the show, Fallon won't be the host. He revealed this to Matt Lauer live from London on The Today Show: “No, I’m not doing the Oscars. It’s an honor to be asked, but it’s not my year.”
Outgoing AMPAS president Tom Sherak threw out the possibility to Michaels and Fallon before alerting Disney and ABC. And incoming president Hawk Koch didn't know about it either. Sherak had recently closed the deal extending the Oscar Show on ABC through 2020. Technically, it appears that Fallon didn't actually get a real offer to host. Which is why it isn't his year.
UPDATE: THR reports that Michaels has bowed out.