iPhone5.com currently hosts a set of discussion forums for the iPhone 5. The forum was launched in October 2010; earlier that year, Apple released the iPhone 4, and speculation was that the next phone, naturally, would be the iPhone 5.
With the release of the iPhone 4S, and with Apple seeking possession of the iPhone 5 domain, it would seem the die is cast for the next-gen iPhone's name. On the other hand, knowing Apple, this could all be just misdirection.
Misdirection might explain the early attempt by Apple to gain ownership of the domain name. In the past, Apple has been quite slow with regard to these sorts of moves. For example, Apple didn't take control of iPhone4.com until almost a year after that device launched. Similarly, Apple didn't seek to take control of iPhone4S.com until several weeks after that device's launch.
So, why would Apple take this pre-emptive measure? It's out of character for the company, and because of that, it doesn't necessarily mean that Apple is doing so now because it is moving again toward a summer release of its new iPhone.
It could, in fact be misdirection, with the new iPhone being called something different. With Apple, it's impossible to know.