(A.K.): The Mathematica site has graduated from beta stage. Perhaps it is now a reasonable request that migration paths be opened for users?
At this time, I will have to decline this. In the last 90 days, there have been 10 migrations to Mathematica (with a 30% rejection rate), and 2 to Computational Science (with a 50% rejection rate).
As noted by Mad Scientist in the comments, we only open migration paths to new sites when there's a demonstrated need for the migration path due to high frequency and volume. At the levels currently visible, this is not the case - handling it with normal flags for moderators should be sufficient.
To put in perspective, consider Arqade and Game Development - these are very, very easy topics to cross the threads in theory. In practice, there is no history of migration from Game Dev over to Arqade, and there have been only 25 questions migrated from Arqade to Game Dev over the course of over 2 years. Thus, in spite of expectations, there isn't a real need for a migration path because it will mostly be unused. The rate is way slow enough that Arqade can handle migrations through flags.
I'm fine to keep an eye on the situation as time passes - Mathematica is pretty young and they do have a high rate of migration over to here. When there starts to be a build-up of migration rate from here to there, then I'll be happy to reopen the consideration of a migration path. Same goes for Computational Science. Until then, no new migration path.
I'm writing this answer to provide both a target of upvotes for people who would like to see this migration option and a space where we can collect questions that would have been appropriate for the Mathematica site. I agree with Asaf's comment under Grace's answer (which received more upvotes than the answer by the way); we need statistics on what actually happens to such questions; if it turns out that many of them fail to be migrated manually (and are either just closed as off-topic or stick around on the wrong site), then the statistics that Grace referred to actually argue in favour of the migration option.
If you see a question that should have been posted on http://mathematica.stackexchange.com, especially if you vote to close a question for that reason, please add it here: