I don’t think we’ll be seeing LTE in an iPhone for some time. Why? Engadget’s review of the LTE Galaxy Nexus by Tim Stevens serves as Exhibit A (the strong emphasis is mine):

And then there’s the battery life. It’s well known that LTE can put a real hurting on phone longevity and that appears to be the case here as well, our Nexus struggling to hold on to a charge in day-to-day use with all antennas firing. We’ve as of yet had very limited time with the thing, but in our 24 hours of intensive testing we had to reach for the charger multiple times. Using Google Navigation with LTE enabled? The battery drained so fast our in-car charger couldn’t keep up, leaving us unsure of which exit to take off the 101.

Stephen Hackett, in response to another article regarding LTE’s affect on batteries, gets straight to the point:

I don’t think Apple will ship an LTE-equipped device until it can ensure the average user can enjoy good battery life with faster data enabled. Most people wouldn’t know how or when to turn off 4G.

In the iPhone 3G, 3GS, and 4, Apple had a switch buried in Settings.app for forcing the 3G antenna off. The 4S has no such setting. As far as I’m concerned, Apple could have removed it from the 4 as well in iOS 5, but they didn’t. 3G hardware has become efficient enough to forego the need of a manual fallback. Wouldn’t it be nice if Apple never needed to have a manual fallback when it comes to 4G? I think it’s better for Apple to let 4G mature, instead of shoehorning it into what is still the best phone on the market.