I don't seem to be able to use http://data.stackexchange.com/ on MathOverflow.
Will this change?
The data explorer now supports MathOverflow. See, for example,
(For those not sure what this means: essentially you can write SQL queries against the (public parts of) the MathOverflow database, and run and save them through a nice web interface.)
It's not there yet, but will be within a few weeks. There's additional work for the SE folks to hook this up, and it's lower priority than the main migration (and probably many other things). See Public dumps for mathoverflow? for details.