yes, you are allowed to leave the airport and visit the World Expo without a visa if your stopover in Shanghai for transit is less than 48 hours.

That's what I thought, but I'm confused because I got a reply from the Chinese embassy that I need a visa because I am not a transit passenger from the US to a third country.
My plane arrives from Saigon (Vietnam) via Tokyo to Pudong Airport, then it leaves Pudong to Tokyo in less than 48 hours, does it count as a stopover in Shanghai for transit?
Thank you!

Ms.Tina, yes, I think it is counted as a stopover, Saigon-Pudong Airport-Tokyo. According to the reply of the chinese embassy, only you leave USA to a third country, transit China is counted as a stopover, I don't think it is true.