Steven Sande at TUAW references this article at the Houston Chronicle concerning the iPhone and LTE. In it, Dwight Silverman says:
Presumably, if the next iPhone comes with LTE, it will use newer-generation chipsets and Apple will have tuned iOS for battery longevity. But I’m not optimistic. I’m also hoping that any LTE phone Apple offers will come with a setting that turns it off and allows users to drop back to 3G when battery life is more important than raw speed.
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.