As far as North America is concerned, as long as they make good on their promise for low ticket prices, I think they could do the following stadium shows:
LA- 2
San Fran-1 (possibly 2?)
San Diego- 1 (?)
Phoenix-1
Vegas- 1
Seattle- 1
Salt Lake- 1 (Rice stadium only holds 33,000, and was one of few US shows to sell out Popmart)
Denver- 1
Dallas- 1
Houston- 1
Atlanta- 1
Miami- 1
North Carolina- 1
D.C.- 1
Philly- 1 (possibly 2?)
NYC- 2 (possibly 3?)
Boston- 2
Detroit- 1
Chicago- 2
Pittsburgh- 1 (?)
Ohio- 1 (?)
St. Louis- 1 (?)- maybe if no Kansas City
Minneapolis- 1
Wisconsin- 1
Montreal- 1
Toronto- 2
Winnipeg- 1
Edmonton- 1 (possibly 2?)
Vancouver- 1
That's 38 shows. They may not ALL sell out, but I still think that seems pretty realistic. Now, if they do another stadium tour in 2010 with another new album, demand would increase. They could do 2 shows at Gillette stadium Boston in 2009, and then do 2 shows at Fenway Park in 2010. Yes, cities like Tampa, New Orleans, San Antonio, Portland or Buffalo will most likely get snubbed and have to travel. But I think that if properly priced, and properly staggered over 2 calender years, with an album released in the middle... U2 could do a decent stadium tour in North America.