Yeah, it was the iPhone 4. Previously on their software the signal meter was tied to some specific RSSI dBm, but that phone had a poor antenna design and dropped calls at higher perceived signal. So they responded by adjusting the meter to better reflect call quality instead. You can enable raw RSSI numbers in the field test menu still.

Also related around that time: Apple pushed an update for AT&T that changed their HSPA indicator from 3G to 4G without any upgrade haven taken place.