Developer Krishna Sagar has found references in iOS 5.1 that refer to switching to 4G when on a phone or FaceTime call. And iDB verified the new iPhone's 4G capability by finding text strings contained "4G" options on all 5.1 IPSW’s.
iDB was able to confirm the following strings appear in this file: var/stash/Applications/Preferences.app/English.lproj/Network.strings
"As stated, our tipster exclusively provided us with screenshots and detailed information from iOS 5.1. To get this information he used iFile on a jailbroken iOS 5.1 install from an iPhone 4. We were able to replicate this and grab the same exact information on our iPhone 4. We were also able to get the same data from an iPod touch. We’re assuming that these strings exist on all 5.1 IPSW’s.
These particular strings are new to iOS 5.1. We have confirmed that they do not reside on iOS 5.0.1."
"4G_ON_CALL_CANCEL" = "Cancel";
"4G_ON_CALL_OK_DISABLE" = "Disable";
"4G_ON_CALL_OK_ENABLE" = "Enable";
"4G_ON_CALL_WARNING_DISABLE" = "Disabling 4G will end your phone call. Are you sure you want to disable 4G?";
"4G_ON_CALL_WARNING_ENABLE" = "Enabling 4G will end your phone call. Are you sure you want to enable 4G?";
"4G_ON_FACETIME_4G_WARNING_DISABLE" = "Disabling 4G will end FaceTime. Are you sure you want to disable 4G?";
"4G_ON_FACETIME_WIFI_WARNING_DISABLE" = "Disabling 4G may end FaceTime. Are you sure you want to disable 4G?";
"4G_TEXT" = "Using 4G loads data faster, but may decrease battery life.";
After seeing the performance of the LTE iPad, the 4G LTE iPhone can't arrive soon enough. Get ready for the 4G times, guys!