Not really. You could always write a browser app for iOS or Android that uses Firefox or Chrome's user agent to have servers return the full desktop version.
If I was AT&T looking for unauthorized tethering folk, I'd focus on the people that are using > 2 GB a month + other various heuristics. Something deff smells funny if someone is using substantial bandwidth and much of the traffic is with a UserAgent like Firefox or Chrome
Ah yes because it's impossible to spoof your user agent string be it with a 3rd party web browser on the iPhone or your desktop browser to match the iOS one.
sure but many websites use the useragent to alter the presentation for the device...this would diminish the experience for the lay user when tethering