Since the release of the new iPad with LTE support for AT&T and Verizon, it seems all but certain that Apple will bring true 4G to the next iPhone as well. Indeed, there have been several rumors to that effect in recent months, and we're getting some more solid evidence today on news that strings specific to 4G calling and FaceTime have appeared deep within iOS 5.1. The news was first reported by iDownloadBlog, and we've now been able to independently confirm that they're there.
Many were surprised to discover that Apple hadn't enabled FaceTime over LTE with the launch of the new iPad. The company had previously elected to keep it disabled over 3G networks because the user experience simply wasn't consistently good enough at that bandwidth and latency, but LTE's significantly improved performance would resolve those issues. Instead, it seems that Apple may wait for the launch of the next iPhone to tout 4G FaceTime as a "new" feature (which it could then potentially unlock on the new iPad via software update).
The 4G calling strings discovered in iOS 5.1 are even more interesting. No network anywhere in the world presently supports integrated voice over LTE (VoLTE), though Verizon and MetroPCS are both expected to begin rolling it out commercially as soon as this year, so it's possible the next iPhone will be among the first wave of VoLTE handsets on the market. Historically, Apple has taken a measured approach to new network technologies — remember that the first iPhone didn't even support 3G — but it may feel that LTE is maturing to the point that VoLTE support would be practical this year, at least on Verizon (which has been field testing VoLTE for months).
Strangely, all of these strings inside the platform use the term "4G" specifically, which is confusing if nothing else — Apple made waves with the release of iOS 5.1 by changing the iPhone 4S status bar to read "4G" when connected to an HSPA+ network. By all accounts, though, Apple is definitely referring to LTE here: one such string, "FOUR_G_SWITCH", contains the text "Enable LTE". And besides, it would make even less sense for the company to enable FaceTime over the slower HSPA+ network at this point.