My husband and I flew back from Japan on August 10th via Delta Airlines and had no issues. The lady at the Delta check-in counter did comment that his extension was longer than they usually are (they made the 751 extensions 48 months rather than 36 months). When entering into America the agent looked at his green card and extension letter, asked us how long we were gone and what we brought back, and that was it. All in all everything was really smooth.