I think it is April 15, south of 60.
No, studded tires are not obsolete. Even the best winter tires are improved with studs.
I used to think things like studs, 4wd, or awd were crutches used by southern military wives, but anymore, anyone driving without awd/4wd with GOOD studded tires in the winter is a damned hazard to themselves and everyone else on the road.
When I used to drive back and forth from Wasilla to Fairbanks in bad weather, I used to leave my tow rope attached to the truck after pulling the first little bubble car out of a ditch. One drive alone, I must have pulled a half dozen folks out of the ditch.
EDIT: Your google-fu is weak, grasshopper:
http://doa.alaska.gov/dmv/faq/faq.htm
When must studded tires be removed?
It is unlawful to operate a motor vehicle with studded tires on a paved highway
or road from May 1st through September 15th, inclusive, north of 60 North Latitude and
from April 15th through September 30th, inclusive, south of 60 North Latitude, except
that at any latitude on a paved portion of the Sterling Highway a person may not operate a motor vehicle with studded tires from May 1st through September 15th, inclusive.