We're hearing from reliable sources this evening that Microsoft's Windows Phone event tomorrow (which we'll be covering live starting at 10AM ET) will be entirely a software story, almost certainly deep-diving on the so-called "Mango" update coming later this year. Of course, all eyes are on Nokia right now to deliver on its new Microsoft partnership, but unfortunately, they don't seem ready to show any Nokia-branded hardware here -- and we won't see any from other top-tier manufacturers like HTC, Samsung, or LG, either. (It's unclear whether any hardware partners will be on hand with existing models to show support for the new version of the platform.)

We already know from announcements dating back to Mobile World Congress in February that Mango will bring some desperately-needed goodies like multitasking, turn-by-turn navigation and deep Twitter integration -- but since CEO Steve Ballmer has recently teased a total of 500 new features in the update, this should still be a marathon of an event, even without new handsets floating around. Tune in tomorrow morning for the full story.