Well it seems the correct method is not to hardcode the path but to use the createlang script. For now... plans in the future include making the DB use a search path to load the file from the correct library dir.
As a work around for this above issue you can fetch version 1.40 of src/pl/plpgsql/src/pl_exec.c. The comments from the last change of the file that caused the problem suggest that this will work fine for tables that haven't been altered or inherited (the reason for the original patch that caused this problem). I have tested it and it works. **DISCLAIMER** This is not the official patch or even suggested patch from the pg team. Just my tinkering.
