That's an interesting point, annieb and vamp, but why should psychological suffering be regarded as different from physical suffering? After all, if physical suffering is bad or prolonged enough, it can lead to psychological suffering too. Also, our brains and minds are part of our physical body. I'm getting outnof my depth here, but it does baffle me why we talk about psychological or mental illness/suffering as if they were somehow essentially different from physical suffering. They are both suffering. It is intolerable suffering, with whatever cause, that is the issue.
If the treatment for suffering of any kind simply isn't available (and sometimes it isn't), then a chosen death seems like a good idea to me.