I totally agree AT&T's tethering and text plans are a scam, I will never pay for them. I already pay $30 a month for an unlimited data plan! Tethering and Text should be included in the data plan.
So for texting I use the Textfree Unlimited app, it is free. They give you a real phone number now, and all texting incoming and outgoing through it are free. It is just like the native Message app on the iPhone, except with more sounds and backgrounds. Textfree doesn't do MMS right now but they are adding it soon.
Not sure if I want to jail brake my iphone for tethering, mainly because the 3G network for my area is very slow. During the day I am lucky to get .5MB and at night I get around 1.5MB downloads. I guess if I had no other option it would be fast enough. But I will not pay for this service!