Yes, AT&T does offer iPhone insurance. It's $7/month with a $200 deductible. I choose their coverage for the theft/loss protection that AppleCare+ doesn't provide. It also protects against out-of-warranty damage, most notably water damage.
AppleCare+ costs $99 with a $79 deductable, and no theft/loss protection. Considering I always upgrade my iPhone annually and the iPhone comes with one year of warranty coverage, AppleCare+ is not worthwhile to me.
I used the AT&T coverage when my iPhone 4S was stolen while vacationing in Mexico last year. A replacement iPhone was overnighted to me and the $200 deductable was put on my next month's bill. Overall I was pleased with their service.