With all the anger towards One More Day (which very deserved IMO) there's something that has been overlooked. Right around the time that part 3 of One More Day came out, there was a rumor posted first on Alvario's comicboards that said part 4 would not only have it where Peter and MJ would except Mephisto's deal to have their marriage erased (which ended up happening) and that the unmasking would also be mindwiped (which also ended up happening) but that only Mary Jane would remember everything. How much you want to bet that's what she whispered to Mephisto? "Let Peter forget completely and let me remember everything."
I would also speculate that it's because she remembers everything that this is why she gains superpowers and becomes Jackpot as rumored. Why? Well, my theory is that because MJ remembers the "real world" and knows full well that the "Brand New Day" world is an "altered world," she is able to bend the physical laws of this "altered world." Based on the preview art we've seen and the "Swing Shift" story, this would give "powers" like enhanced strength, agility, speed, and (she apparently survives a punch from a surprised and apologetic Spidey in one of the previews) durability. In other words, she's basically become Trinity from the Matrix in a disco outfit instead of biker leather.
However, the poster who first posted the rumor also added that Mary Jane can tell Peter what reality used to be like because it would cause a "timeline shockwave" which would ripple through all of reality. bet that at the end of the Guggenheim arc, we're going to see Mephisto show up before her and tell her that if she thought she could just tell Peter what the world used to be like and get him back that way, she can, but it will result in fracturing all the other realities. In other words, if she tells Peter that this isn't how reality is supposed to be and that they are supposed to be married, then she could end up destroying the universe.
Of course, you know sooner or later that the truth is going to come out because having something like that is going to be followed up on. Thus, Marvel has essentially planted a seed for their own "Crisis" and will get crossovers between various realities a la "Zero Hour" as a result, which I imagine would be the various Marvel heroes form various "worlds" banding together against Mephisto and various villains from other worlds to restore reality back the way it was. After all, Joe Q is on record saying that "If we ever had a crossover between the 616 and the Ultimate universe, you'll know that we've run out of ideas." Which, of course, given how he's blatantly lied in the past translates as "we've got one already in the works."