I was able to take a closer look at the Forest Department Dormitory when I visited Kanneliya last week end.
The Kanneliya Conservation Center (the whole complex) seems to be managed better than some of the other forest department facilities. The dormitory had four rooms, a five bed room, 13 bed room and two 16 bed rooms (all together 50 beds). every room had attached bathrooms which were very clean and well maintianed. The entire facility is well maintained and clean.
It costs just Rs. 250 per person. There is a cook to prepare meals, you have to take rations and pay for gas. In my opinion, this is the best accomodation option if you plan to visit Kanneliya forest. You have to make reservations at forest department head office at Baththaramulla.
Check this thread for the forest department contact numbers: http://www.lakdasun.com/forum/index.php?topic=300.0
See some pictures below