why? you put the auto check for emails on manual and thats it. there is no data traffic unless you initiate it by starting safari or whatever.
i use my iphone unlocked on e-plus in germany and i dont have a data plan at all, until now i just need a lot of data when i am in wifi range anyways. i might get one though.
there have been data flatrates in germany at least for a while already, ranging aroung 25-30 euros on o2 and eplus. its for 5gb so not a real flatrate but that should do it i think. i very much doubt the iphone will be introduced anywhere without at least the option for something like that.