They have noted from monitoring cash remittances to try and prevent terrorist financing that actually, migrants send more money to developing countries than international aid agencies and investors. They also noted that migrants who move to more liberal countries can influence the culture back at home, but pushing for increased regulation or transparency.
Apparently, this current economic crisis has proved a bit of a boom for some African countries like Nigeria, where a lot of expat professionals are now moving back after living and working in London and New York, bringing their skills and experiences with them (and hopefully pushing for development and less corruption).