The necessary LTE chipset for Apple (Qualcomm 9615) wont be in mass production until the end of the year. So the new phone wont be LTE.
What I worry about is Apple waiting until next Sept. (of 2012) to bring LTE to the iPhone, even though there will be a capable chip available much earlier (and there will be a ton of LTE devices on the market that have good battery life using this same chip). On the flip side, Apple could release the iPhone 6 as early as May or June of 2012, so whats the purpose of getting the iPhone 5 if the LTE phone is just 8 months away? Its a weird situation they're in with their product release cycles.
I wouldn't worry about it. There were plenty of 3G phones on the market before Apple took the plunge and it didn't affect them negatively at all. They basically waited until they could get it right, and this paid off. The change to LTE will be the same. Apple needs to stay the course and wait on LTE until it is power efficient and until the network is much more widespread. It's too immature right now, and most users are just fine on 3G speeds.
Oh, and there is absolutely no way that an additional iPhone with LTE will be released 8 months after iPhone 5. Not unless they had a major problem with the 5 and needed to kill it off quickly. Other than that, I believe Apple has now in effect changed their iPhone refresh cycle to the Fall instead of Summer.