Wifi does NOT disable a 3G connection. Most data will move via wifi if it's enabled, connected and capable of moving data but it doesn't turn off 3G. If any of those aren't valid, it'll eventually fall back to 3G and some functions of the phone use carrier data regardless of the wifi setting. One easy example is sending stuff via MMS. It's still data but it won't use wifi even if it's enabled. If the phone has any problem with the wifi connection, it may still show it as connected but it's using 3G. As far as I've been able to tell, the "wifi bars" simply indicate that it's gotten an IP address from something.