You can mirror joints no probs and skin weights.

What I do is create your model completly then add the spine, one arm and one leg, dont have them joined to together at this point, name all hte joints, then mirror the arms and the legs over (using mirror joints on the skeleton menu (I think) replacing in the name left for right or right with left depending on what you've called it. Parent the arms and the legs to the respective joints in the heirarchy, then paint weithts on one side ofthe body and mirror them over.