This same file was found by BGR last week in an 'internal iOS test build' provided to carriers for testing.
We've since discovered the same LTE plist file in at least the last couple of developer builds of iOS 5. This LTE.plist file was found only in some of the builds (GSM iPhone 4, CDMA iPad 2, but not their counterparts) and it was related to Apple's internal "Fieldtest" application. It raises the question whether or not the "internal iOS test build" was anything special or if it just mirrored the general developer release.
Rumors that Apple might actually have LTE ready for the iPhone 5 were reignited last week when Engadget posted a picture of LTE equipment that was recently installed in an Apple Store.
Apple has said that "the first generation of LTE chip-sets force a lot of design compromises with the handset, and some of those we are just not willing to make". Thus it would seem that the only hope for an LTE iPhone 5 is if Qualcomm was able to accelerate production of its second generation chipset.