I've been thinking this too, and here's why I believe it could be their last;
1. They've struck it big time playing stadiums in America, the first time in over 13 years, they couldn't do this during Elevation, and Vertigo. This would be a HUGE accomplishment and the finishing touch on that. They've proved that they can come back swinging bigger than ever.
2. With Bono getting older, @ 50 years old, maybe it's almost time to retire the stage antics and movements and focus about good health. He nearly came to being paralyzed, he'll serve the tour out at the best he can, but in some retrospect, he's probably afraid it could happen again and maybe not get the full enjoyment out of life.
3. Creativity, where do we go from here? We asked ourselves this with "Bomb" -- surely "No Line" is a quasi-reinvention of the band itself, but it borrows from previous records and doesn't really give us a solid direction of where they're headed next, unlike how Zooropa birthed Pop.
But then again, time will tell. Maybe we'll have the same laugh we did in 2001/2005 about how we were wrong that they're got nowhere to go.