The purpose of this study is to understand the effects of intrathecal ziconotide (an experimental pain medication) when the dose is slowly increased over a 3-week period in patients with severe chronic pain. During the weaning phase, the study will also gather information about switching from other intrathecal or IT medication (slowly pumped directly into the space around the spine) to other systemic pain medication (by mouth or through the skin using a patch). After being weaned off current IT medication, patients will be placed on IT ziconotide or placebo (non-active substance) as well as being allowed a stable dose of systemic pain medications.