More data visualization: migration

The International Organization for Migration has a cool interactive map on their website. Click on a country to see where its people are emigrating, and from where other migrants are immigrating. Fascinating to play around with.
It would be great to see something similar for intra-country migration too (though that data is more difficult to come by) to map urbanization and other trends. Or to filter by other data points, such as gender, education levels, economic indicators etc. And then to track the whole thing over time.....