MathOverflow has a large supply of reasonable to excellent mathematics questions, which have been closed for not being "research level".
Should we migrate some of these by hand?
I don't see anything wrong with asking questions that:
The goal of the SE model is to be an authoritative reference for people of all levels. I say migrate (by hand of course).
However, when it comes to providing an answer from mathoverflow/other site, I think the best practice is to just link to the answer. This way, the original author gets the credit.
This is not a good idea; a similar question was asked for Game Development-related questions at Stack Overflow being moved to the new Game Dev SE Beta.
Any questions asked at MO and closed can simply be re-asked here. There is no need to over-complicate things with possible migration issues.
This is even more complex than the Game Development case, because the Math Overflow database is independently owned apart from Stack Exchange.
There is no software facility for migrating questions from MO: at present, as Justin says, it is a completely separate system that does not communicate with the Stack Exchange 2 sites. And of course, right now, the only people who could be reask questions here are those who are invited to the private beta.
It's also unwise for MO to count on this proposal being a place to redirect questions in the future: we're in the second day of private beta, and the site might not make it all the way to live.
Generally, I don't think "migrate" is a good word to use in this context, unless MO does become a Stack Exchange 2 site; people are free to ask questions here, whether they have been closed at MO or not.