Like others carriers including Verizon, AT&T charges a $20/month rate to enable tethering your mobile phone to any of your other devices. To avoid this charge many AT&T iPhone and Android 2.2 users have worked around this by jailbreaking (iOS) or rooting (Android) their phone to add applications that let them tether at no additional charge.
AT&T has started sending out an unambiguous message this week: they know about it, and they want it to stop.
Strongly worded messages have started going out to their customers who use unauthorized tethering, telling them that they can either stop tethering, or upgrade to a $45/month DataPro 4GB plan ($25 for data, $20 for tethering). If offending customers don't respond in time, well, there will be consequences: