It is not a secret that AT&T hates their customers. If you managed to cling to that old unlimited data plan, then they particularly hate you. According to some online reports, its seems that they will exact their revenge on those people by raising the price of the unlimited data plan by $5 to $35.
Keep in mind that this is for people that have managed to remain grandfathered in the plan. The actual plan was stopped in 2010 and this is the first price increase in seven years.
Like many of the other carriers, AT&T managed to throttle unlimited customers when their data exceeds 22GB per month. Of course, AT&T is not the only one. Verizon increased their unlimited data by an extra $20 per line.