I have a couple questions about this. First, have the people at mywi or tetherme had any input about this? I would think they would be on top of this and have some kind of blog or forum to address this with updates and info.
Also, I have not tethered yet but my thinking is that it is detectable which hardware is connected to the internet and they may be able to monitor that. If I go to yahoo.com with my iphone i get yahoo for iphone, if i use my ipad I get the yahoo for tablet. Similar for macrumors. They gotta know somehow, plus when I hit some sites from my iphone I get the popup asking if i would like to download the free app for the site from the app store...... so somewhere along the line someone knows exactly which device is pulling info..... I wonder if you could use a mac spoofer from cydia on an ipad and spoof your phone mac... might cause a conflict though and then you would get the iphone view of webpages...