I think the lack of leaked parts is probably Apple tightening security. Jobs was pissed last year when the 4 was known to the public before he got to show it off on stage last year.
The fact that iOS 5 won't be ready is really not too concerning; I think if Apple were to launch a new iPhone, it would still ship with iOS 4 because as time goes on it becomes more prudent for Apple to decouple the phone and OS release times. With the software now running on 3 distinct product lines, and potentially more in the future, it makes less and less sense to make the OS release alongside a phone.
However, I don't know if WWDC will bring an iPhone update. Even if there was one, it would be a modest bump at best I think. Apple likes the 4's design, so they'll probably try to get as much out of it as they can. I'd say it's a 50/50 shot right now, but I can say that it would be a mistake to not update the phone by the end of July (at the latest). The mobile market moves too quickly for Apple to try to force its own time scale on customers. Some people might be willing to waiting months for an iPhone, but most probably won't. When other models offer bigger screens, more options, and up-to-date features, who can blame them?
