Mid means in the middle of
Medio is a term used to indicate that something is coming, but it is not yet exactly known when it is. By saying that a certain project is ' Mid 2012 ready ', it is indicated that it is ready in 2010, but there it is not yet known whether it may 2012 ready, or September. Generally speaking it is assumed that mid is roughly in the middle, mid 2012 is therefore not February or November, but more near July/August.