I have $GOBIN set to /Users/r/bin but go install always puts godoc in $GO/bin, which is
not the same thing.

% go install -n code.google.com/p/rspace.cmd/doc 2>&1 | tail -1
cp $WORK/code.google.com/p/rspace.cmd/doc/_obj/exe/a.out /Users/r/bin/doc
% go install -n code.google.com/p/go.tools/cmd/godoc 2>&1 | tail -1
cp $WORK/code.google.com/p/go.tools/cmd/godoc/_obj/exe/a.out /Users/r/go/bin/godoc
% 

Notice the different directories