I've got two general thoughts on that. One is what I recommend in the partitioning howto -- ln -s /usr/local /opt. The other is to create a subdirectory /usr/local/opt and link it instead, as the top-level /usr/local directory otherwise can get somewhat polluted. This is mostly a question of asthetics.
I generally vote against dedicating a partition to /opt -- there's a point at which you've chopped things up too much, you're better allocating the space to local and divving it up among well behaved local stuff whether it's in /opt or /usr/local.