Jack and Miranda... no, ma'am, I can't ship that.
I think it's a bit too much of a bridge to cross to think that romance could flourish there. They're very similar women; both engineered in a sense for greatness, both driven to be something they don't want to be, both strong women capable of defending themselves against pretty much anything, blah blah blah. But Miranda has a rather overly-mature, jaded woman's head on her shoulders, while from an emotional standpoint, Jack is still a free young girl in many ways, and it makes them too emotionally different to I think thrive in a lasting romance. Plus they've already drawn a line in the sand that romance would be hard to cross. So no, can't see it myself.
When you think about Miri and what we know about her and intimate relationships, it's a wonder she can manage it with Shepard, let alone Jack. Miranda is certainly interested in people sexually and in dealing with her own physical needs, but emotionally is only really connected to her work. Facts and figures and barriers to be broken. She lives for what she does; it completes the missing part of her, fills the void left by the bang-up job her father did of raising her as not so much a child as a living, breathing idol to his own vanity. And she knows that too... that's got to be hard, knowing a parent has never really seen you as more than an investment. Because of that, Miranda might even be seen by others not as a cold fish, but just emotionally retarded; emotional bonding is just hard for her because she relates it with deception or with the eventual discovery of unlterior motives in others. Career won't hurt you like people will, so she pushes people away - like Jacob; obviously interested in her for more than her looks and place in society, but she pushes him away... now, can we argue that that is because he didn't fulfil her prerequisites for a casual partner, or because he scared her with emotions she expected to burn her later? Working for Cerberus can't help either, when no-one is ever straight about what they mean.
Jack's approach to intimacy is footloose and fancy-free, but part of her is looking for more - she's seen what other people can have, and wants it for herself, although she's also frightened of being burned yet again by someone that gives with one hand and take take takes with the other, so she winds up blowing hot and cold and giving mixed signals. So while Jack can easily seperate emotion from casual encounters, there is emotion there nonetheless. I think that's Miranda's problem; making those bonds, wanting to be loved, maybe even wondering if she deserves it. Jack is pretty similar, but she doesn't stop, she doesn't throw herself into some other occupation. Sure, Jack says she doesn't get involved because people let her down, but we know it's a front - Jack wants to be that happy person deep down inside, so she keeps risking getting her heart stomped on over and over and over, in the hope next time the next person will be real. Jack is hopeful, brave, toughened up for sure, but she's not let it freeze her heart over no matter how much she puffs herself up and says you're too needy. With Miranda, it almost feels like she's resigned herself to having to do things without attachments... she's figured out the numbers, and it's just not worth finding out someone could hurt her emotionally as much as her father did. In a way, you can feel something there for her to a degree we usually only apply to Jack; Jack never had a parent, a primary giver of love and trust and comfort, to betray her so badly... and you can't miss what you've never known. Miranda though had to discover for herself what her father really was, most likely in her youth. And that must be absolutely devastating.
It's because of this I can see them one day becoming able to interract. They'll always be stratified; Jack the irritating younger sister and Miri the older, obstinate, emotionally-mature one, and they'll always bicker or have all-out feuds, but I think they could come to really understand one another. I'll wheel out my analogy of Herzog and Kinski - best of fiends... sometimes they really do want to kill each other, but what stops them is the fact they can see enough things they can admire or outright like to deal with the occasional friction.
So I'll leave the Jack/Miranda romance to that picture that got posted in the AP a couple of days back: entertaining on paper, but nowhere else. Not for me at least. That picture did show that there's a reason Kelly's on the ship though...