given how poor AT&T is at managing their network i can't think they'd be watching people's data transfers too closely, but who knows. the only thing that could alert them is that the
ipad consumes more data than the iphone. so a sudden increase in usage might tip them off.
that being said, i used mywi just fine in a few airports with my iphone plugged in nearby (since it drains the battery like a vampire). i looked at it this way, if i wasn't surfing on my ipad i would have been surfing on the iphone anyway. my data usage shouldn't have gone up that much.
if at&t could get their network together it wouldn't be much of an issue. and there's also hope some other carrier might start supporting the ipad directly since it isn't carrier locked.