You don't say where you are.. but I shot some very effective portraits on the street outside a street cafe last week, using the artificial light of the cafe to light the subject, shooting raw so that I could correct for strange artificial lighting colour casts when I got home, and with the streetlights and vehicle lights as nice bokeh balls in the background. I'm afraid none of them are uploaded at the moment. Wide open will capture the best of the available light and give a nice shallow depth of field.
The best night portraits always seem to contrast a well lit subject against an interesting background. If you're using flash (preferably off-camera) you can pick any spot with a nice background. If you're not using flash, see what artificial light is available to work with that also has the best background behind.
