I ran a speed test on my (throttled) AT&T 3G connection on my iPhone 4. 5 bars, perfect signal here. 0.03 Mbps down and 0.02 up. It's absolute bullshit.
That's not entirely true for T-Mobile, actually. You still connect to 3G towers even after throttling, but you are slowed down to "2G" speeds. In practice, the 2G speeds are much slower than EDGE, but slightly faster than GPRS.
And that's the trick: AT&T implemented throttling just after all the dedicated iPhone users upgraded to an iPhone 4S and locked themselves in for another two years.
You didn't have to lock in. If you paid full price for the iPhone 4s, then when you first plugged it to iTunes, you got a "congratulations, your phone has been unlocked" message, and the phone stopped being carrier locked. This worked since iPhone 4s release day, before the 'official' unlocked phone shipped.