iPhone is synced from one Mac or PC — sort of. We're used to floating our Bluetooth-enabled mobile phones among Macs and using iSync (along with .Mac sync) to keep all the Macs and phone in sync. A change made in any one location updates all the others. At first this did not seem to be possible with iPhone — we thought we would have to pick a master Mac and stick with it. But we've since discovered that we can set iTunes on another computer to merge Calendars and Contacts with those already on iPhone.

But the process of merging calendars and contacts can be very slow, especially if you have thousands of calendar entries or contacts. And, iPhone forgets its sync settings at your previous Mac or PC, so you have to repeat the merge process every time you move from machine to machine. So while you can sync it among multiple machines, in practice it's something of a hassle.

Music, Podcasts and Video likewise sync only from one Mac, but there is no merge feature — each type of is completely replaced. But like other iPods, you can at least transfer purchased music and videos from iPhone to your other iTunes-authorized machines.

The upshot is that you can sync your contacts and/or calendars at one Mac or PC (perhaps a work location), and music at another (such as home), but there is not a true multi-device sync. This is an odd omission, since Apple's iSync tool supports multi-device sync with other mobile phones.