Well, I understand that this is an old thread but it may be of use to others in this forum. I was driving my 2009 Forester tonight, my wife sitting next to me, and the windshield made a loud "cracking" noise. A long crack appeared across the windshield all of the sudden. There was no traffic nor pedestrians whatsoever on the street. The crack appeared at perhaps four inches above the base of the windshield, and all the way across.
This windshield doesn't have a single chip nor scratch, and this crack was caused by the hot defrost-air as follows: Ambient temperature was around -21 degrees F, heat was set to maximum, heat direction was set to both defrost and cabin (one click from full defrost), approximately 30 minutes of highway driving, perhaps ten minutes of street driving, and the blower was set at 4 (maximum).
You don't have to believe my story if you don't want to, but it's true. I have full insurance coverage (insurance pays for the windshield), so I plan to wait until spring to have it replaced.
Assuming there is not a chip anywhere along the crack line, this is absolutely covered under your warranty, don't waste your money on insurance!
Subaru covers stress cracks on windshields. The deciding factor is they will look along the line for a chip. If no chip, you're good to go.
I delivered a '10 Legacy to a customer 2 weeks ago and 3 days after the girl bought it, she got a stress crack that started in the lower corner, and went up and right across the middle. Covered under warranty.
Then last week, I had a customer call me about the exact same thing on his '07 Certified Pre-Owned WRX that I sold him two months ago. Covered under warranty.
I wouldn't let that deter anyone reading this from buying a Subaru, but yes, it happens occasionally.
But please note this post if you have a stress crack and still have your Limited Mfr Warranty, don't waste your money!