Apple's MobileMe for the iPhone was one of the first services offered to make this a reality. But MobileMe costs extra money, and yet it's still not always up and running.
Now here comes Google's Android platform, and at least one (or is that every?) Android phone out there that apparently keeps the local calendar in continuous sync with Google Calendar on the Web, at least according to Tim O'Reilly. Quoting Tim:
"No need to sync address book and calendar. Everything's always up to date."
And unlike MobileMe, there's no extra charge for this.
UPDATE: And as reader John Gordon points out, no Android required here. Just Google Calendar and Google Sync, on any mobile device they already support. Here all this time I didn't realize it!