It's not harder to implement on the ipad, I don't know where u guys are getting that bthe ipad has the exact same internals as the iPhone 3gs and likely the next gen iPhone. I would guess that's it's taking longer because (1) they are implementing ipad specific software features that will make it a less iPod touch like and a little more useable in it's larger form factor, and (2) because they are timing it to coincide with some hardware revision, whether it's a small price drop only or maybe it gets some new features like front camera.
Here's how it works: It all gets implemented, then a dozen iPhones are given to the test team, which will spend some weeks testing and testing and testing and testing with all kinds of applications, all kinds of user behaviours, and so on. And then the iPhone version is released. And then the same testers get a dozen iPads, and they spend some weeks testing and testing and testing and testing, and then the iPad version gets released.
Unless someone at Apple is so confident that iPhone and iPad behave absolutely the same that they think running tests on an iPhone successfully means the same tests will work on an iPad without any testing needed. I don't think so.