Snap.
I finally had to give in. I gave up my iPhone unlimited data plan, which I kept since 2009, for a 3GB plan. As people were saying, the unlimited plan was not unlimited any more. I experienced a throttling in download speed, once I hit the 3GB threshold. It went down to around 150 Kbps level, while the upload speed didn't seem to change. It is not even enough for downloading an email nowadays.
I thought it was their cheap strategy, but I decided it would be better buying additional 10GB for $10 than coping with the throttled downloads for one more week. AT&T seemed to be pleased with my move. The speed immediately went back to normal once I completed changing the plan.