Current legislation requires a 2-physician consensus on terminal illness status of 6 months or less to live. Can this change? Absolutely. Often? Not really. I've heard many stories of 6 months to live lessened to weeks or days, but not the opposite. The process has the patient approach the physician. The physician has a right to say no. The patient takes the medication, of which the physician only provides a prescription for. Each year, in OR (and now WA), many more people die of the qualifying diseases naturally than by the barbiturates prescribed. The process isn't abused, the physician isn't playing God, the law is sound (and provides no legal recourse against a physician), and the patient makes the decision. Not letting people die the way they want is cruel.
As for your psycho- versus physiological question before. Depression is curable, although it can be tough, whereas a terminal disease is not. Common sense says the patient should exhaust all options available before granted the choice to die.
And lets not forget, we already have assisted-death. Removal of feeding tubes, removal of ventilators, administration of narcotics (which have a high indication of death at certain thresholds) just to name a few.
*All this is based on my views of PAS, not euthanasia.