A few weeks ago, it was reported by Mac OS X Leopard beta testers that Leopard's mail included notes feature akin to the Notes program on the iPhone. The optimistic, and somewhat logical, hope at the time was that notes on the iPhone would sync with notes in the new Leopard version of Mail. In fact, note syncing was even listed at one time in Apple's feature list for Leopard. Unfortunately, that doesn't seem to be the case.
According to a review of Leopard published by ITworld, though the Notes in Leopard and on the iPhone make look the same, smell
the same, and perhaps even taste the same - they don't sync. The review states,
Apple Mail 3.0 has also seen numerous improvements ... Mail also now has support for to-do lists and "notes," in which you leave messages to yourself. Mail seems to be an odd place to stow this information, and the to-do interface in Mail is poorer than the one in iCal. Although Mail's notes look a lot like the notes on the iPhone, the two don't sync.
Now, this doesn't mean they won't ever sync, but they don't at this time. It would seem almost incomprehensible for two clearly related programs operating on versions of the same operating system to not share/sync data. The clear assumption, based on common sense and Apple's previous inclusion of note syncing in the Leopard feature list, is that future updates to either Leopard or the iPhone's firmware will provide this functionality.