I'm sure some tweaks will be made, but I wouldn't place too much weight on the fact that the iPhone can be upgraded. That was one of the main things people said when the iPod came out, and was revealed that its firmware and operating system could be easily upgraded. They said that the simplicity and lack of feature in the original iPod could easily be expanded by Apple.
Unfortunately, it never really worked out that way. Apple does release what amounts to bug fixes and minor changes in existing iPods--but to get any real updates you have to shell out for a newer iPod. I'm not saying data sharing will not exist in Leopard. That seems only natural since it will have system-wide ToDos and all. But don't expect miracles.
Yeah, I remember OS X 10.0, and that was what, almost $500 USD ago? My point isn't that they should charge for upgrades, but that buying the iPhone now and expecting it to radically improve and evolve over its current limitations is perhaps a bit optimistic.