kmc, the water purity issue has sometimes brought about huge and passionate discussions. Many people still treat or filter their water, and the Forest Service advises people to do that, but there are only a few places in the Sierra that scientists have actually found enough contamination to be harmful. They are in drainage from cattle grazing areas, and areas below livestock holding sites (pack stations, etc.)
You can choose what to do yourself; the choice is yours.
Here are links to some discussions and studies in the past:
Water discussions (Drink untreated, or filter/treat/boil/etc.)
Water and Giardia around Mt. Whitney 60,000 Liters of Water Consumed -- Untreated Filter question (links to studies, etc.)
