The marriage is officially kaput. For now. Probably the most disappointing thing about all of this is how bad the writing has been. JMS has said (I believe so, anyway) that he didn't like the idea behind "One More Day" and from the way he wrote it, I believe him. I've also never bought into the idea that Peter and MJ are somehow "fated" to be together, so Mephisto talking about their love being "holy" and all that crap just falls flat.
That said... Anyone who believes that the marriage is dead is kidding themselves. The end of OMD sets things up perfectly to make sure that MJ and Peter not only get back together, but that they need to get back together. Harry's alive (and divorced from Liz, apparently). That in and of itself should prove that this story is not going to last. Harry Osborn did nothing but screw up his life every chance he got. If he's really been alive all this time, it means that he's just had more time to screw up his life. He's going to end of in a situation where he's "better off dead," if you know what I mean.
Add to that the fact that things between BND Peter and MJ are "frosty" and it's very, very clear that their relationship is far from done. It is my firm belief that BND is designed more to reinforce the marriage and its need to move forward (as in children) than it is to "end" the marriage. The one thing that will last from all of this is that Peter's identity will be secret again, and that's fine with me. Talk about a genie that needed a bottle... there it is.
I may be right, I may be wrong, but as bad of a story as "One More Day" was, it seems clear to me that its purpose is NOT to serve as a cop out. If it is just a cop out, just a crappy way to rid themselves of the marriage, then I'll be more than disappointed--I'll be surprised.
One more note, specifically to Marvel: If people buy more Spidey books starting with "Brand New Day", it's not because of the marriage being gone--it's because with good writing, the Spidey books might actually be readable again.