I wanted to post a link referring to recent progress on adapting Symbolics keyboards to modern PCs. Hans Hübner announced that the kbdbabel project (apparently based on 8051 microcontrollers) was able to be adapted to support the slim-line (Rev. C) Symbolics keyboard, adapting it to PS/2 protocol.
My own work in this area ground to a halt pretty soon after I received the ADB adapter from DKS. I'm happy to see that others have been able to make progress!
UPDATE: Rainer Joswig has commented on his experience using the kbdbabel adapter to connect a Symbolics keyboard to a Macbook running Mac OS X.
UPDATE 2: Hans has developed a USB adapter solution.