I don't think I had a cold sleeping night at all through the Sierra, but it did get below freezing each night. Definitely not down to 15 F. Being in the trees, in a tent (even a tarp tent), eating an insane amount of food, and supplementing the sleeping bag with the thermawrap was just fine for me. The only time I wished for more insulation was after I left camp in the morning, before the sun came over the mountains. The temperature swings between night and day were like... well, night and day.
For current conditions, I'd recommend getting on the PCT facebook groups (the official Pacific Crest Trail group, and right now the PCT Class of 2012 is a good one). I haven't been out there since 2010, so my info may be a little clouded by memory.