” the phone apparently now “[utilizes] whichever network band is less congested or has the least interference for the best signal quality, regardless of actual signal strength” — in other words, better call reliability” (via iFixit iPhone 4 teardown)
Let me translate that: you leave your phone on dual network and hope for a speedy 3G coverage which seems to have full bar signal; but your phone takes the opposite decision because your location is too noisy for a reliable 3G connection, as your operator never gave a damn about network architecture. So, you fall over GPRS or EDGE. Which, I give you this, will make you really mad at the end of the month when paying the bill and keeping in your hands a beast that has no quality fuel to run on.
Instead, the poorer the architecture of your 3G operator, the slower your iPhone 4 will walk.