I was thinking this same thing the other day. I work for AT&T, so I brought this thought up to my manager. She made a very valid point that you and I didn't think of: The 5GB package includes tethering. The $30 Unlimited offered data only on the device.

With that said, it makes sense (in an odd way), that customers get slowed down at the 3GB.

Note: My theory; AT&T is only trying to slow down iPhone users since they're HUGE data hogs. They just haven't found a smart way of doing it just yet. Give them some time. They'll work out the kinks.