Apple has never been much bothered about users who find the interface 'boring', they design for what they perceive as ease of use. Baring in mind the typical interface on cellphones before the original iPhone shook up the market, and how far everyone else has had to go to rethink the way users interact with their cellphones, I think Apple got the UI pretty much right for the vast majority of the market.
I also don't believe that the majority of users much care that this year their iPhone looks the same as it did last year. They don't look at the thing, they use it. And since the way they use their iPhones is pretty much the same, there's no real need for a wholesale change in UI. Nor would Apple be likely to pander to a vociferous few amongst their customer base and put the UI through a wholesale redesign that they don't think the majority of ordinary users would much need.
What changes 4.0 will bring would, I think, be much more tailored towards the new features and resources added into the iPhone since the first model, and be specifically intended to extend the ease of use factor into the growing complexity of the device. Better organisation and access to apps, perhaps, along with improvements made possible by a better hardware platform.
There's nothing much wrong with the iPhone's UI, and in the context of a mobile device operated by touch, much that is right with it.