i used to live in blantyre, and climbed a line in the middle of the chambe east face on mulanje. it was a beautiful route, in a pretty stunning area.
climbing on mulanje is reasonably serious and committing - you're in the middle of nowhere. the rock is generally good, but often vegetated and/or grimy, and you'll deal some pretty long run-outs.
try to get hold of the Mulanje guide by Frank Eastwood - it has a bunch of route descriptions, of varying accuracy. most of the good stuff is on chambe, and the west face is supposedly the biggest wall in africa (the alard hufner website posted)
i also did a bunch of cragging around chiradzulu, near blantyre and on the viphya plateau. the northern lakeshore (around and above nkhata bay) has a plenty of bouldering potential (and the landings couldn't be more fun - splash!)
there's plenty of rock dotting the landscape in malawi, but its all real bush-climbing - you see something interesting, have a look around, climb from the ground up, and hopefully find an easy way off.