PalmTree: Hello, I am currently studying for a masters degree in GIS @ unilorin. Reading through your post one line caught my attention, you will like to go into academics (lecturing) right? Now, GIS is a professional course that exposes you to how geographical/spatial data can be used to solve human problems with great precision. Because it is professional, most specialists do not take on PHD studies and I doubt if any Nigerian university offers one, so if you venture into academics, you might most likely hit a 'glass ceiling' mid way into your career.
But if you decide to go professional, then you must make certain of one thing, just like other professionals you need to be good at what you do and then am sure you will make it because gis is increasing in its relevance to most industries ranging from oil, telecoms and banking to urban planing,environmental management and surveying.
As for the universities offering pgdgis/mgis courses, you have a lot to choose
from unilorin, unilag, u.I etc.

Duration for pgdgis is 12months with a maximum of 18months and the cost ranges from 200-250k
Goodluck to you.