I don't think they'll have to charge for it..
a) they've probably got tons of free HD space with all the 1000s of computers they have networked together running the SE.
b) Only a very small % of users will actually USE anything like 1Gb.
c) They're running context-based (adsense-type) ads in the emails. Which probably means it will be a bad idea to email your grandmother about her "breast operation"
