I was just surfing another Kotor fourm and this person *claims* Kotor III is anounced but I just take it as a rumour. Have a look for yourself any way.
Just take this as a rumour because I can't see any offical source from Lucasarts.
This person claims:
Kotor III Rumour
Kotor 3 Rumors: Well its true! There are announcements from lucas arts of a new knights of the old republic game. They are already at a finish for the game and is planning for a release in 2006. It will hit the new big X Box 360 with all the new amazing graphics. You will be able to have more features in the game as such as ruling your own empire. More Kotor 3 games news will be announced whenever Lucas Arts does.
source:
http://www.starwarsclub.tk/