As far as demand in the U.S. goes for U2 concerts (assuming tickets are priced similarly to Vertigo tour):
U2 has built up enough demand in the USA to play several large arena shows (15-20k occupancy) in large cities like L.A., Chicago, New York, Boston, etc. In cities like New York, Chicago, L.A., U2 can easily do 5+ total arena shows per tour, which is about 80-100k total attendance in those cities. This means that in those cities, U2 could probably get away with doing 1-2 stadium shows, if they ever attempt stadium shows in the USA again. However, stadium shows in most other U.S. cities would be impractical, as U2 usually play 1-2 arena shows in most other cities (Like Dallas, Minneapolis, Phoenix, Seattle, etc).
I doubt U2 will try stadiums again in the USA since the popmart tour left many shows half empty. It would sure be awesome if they did though!