Yeah, it's just a rumor for now, though the tabloid that the story first ran in isn't considered reliable in 2ch wotas' eyes. But the same story was picked up by a gossip TV show.
I'd rather wait and see what happens when March comes, whether it'll be confirmed or not then.
Though if the rumor's true, I wonder if Miki would still want to continue singing with another label after the marriage. I could see her joining avex, for instance. Or maybe she might want to quit singing. I dunno, but I'll support her either way.