PIM sync is what I’d most like to see from iPhone .Mac syncing, both because it means new PIM data (stuff entered into the iPhone) would be immediately duplicated (the ‘data doesn’t exist until it exists in two places’ philosophy) and because that data would be immediately available to others (I’m thinking in particular of my on-line calendar, which I use to communicate my availability, appointments and whereabouts to quite a large number of people). As well as PIM sync though I use .Mac services for plenty of other stuff, some of which it would also be good to have play nice with iPhone:

1) Sync of bookmarks, keychains, Notes and Mail accounts, rules, signatures etc. ‘Notes’ I guess fall into the ‘PIM’ category, and I’d love to see syncing by .Mac. Keychains also, especially if the 1Password guys come up with iPhone Safari integration (yeah, I know the SDK T&C may prohibit this, but we can dream, and let’s wait to see how the third-party limitation thing pans out). Not so bothered about bookmarks, Mail accounts and signatures, which change less frequently and wouldn’t benefit a great deal from wireless sync.

2) Nice, simple transfer of photos to the iDisk web space. Works a treat from Aperture and iPhoto and similar nice’n'easy-transfer of iPhone photos would be a pretty cool way to quickly share them, imo. This would involve transferring a fair bit of data though and so with current iPhones (which can’t do simultaneous voice and cellular data transmission) would probably only be feasible over Wifi. That’s a pretty big restriction, so best to wait for 3G for this?

3) Back to My Mac. This of course has two bits: file transfer between two Macs and screen sharing (VNC). To be useful file sharing would probably require significant changes on the iPhone, not least an exposed file system, so that the user could keep everything organised, and probably the ability to open associated applications by navigating to and tapping a file. Even Mac-to-Mac, screen sharing is more wow than useful for me and, like file transfer, suffers from extreme unreliability – apart from over a local network, it seems to be in the lap of the gods as to whether a connection can be established. To be honest, I’d rather Apple concentrated on getting Back to My Mac working properly with my Macs than started trying integrate iPhone into the system.

4) iDisk file storage. Useful and reliable (if slow) for file transfer between Macs but doesn’t really accomplish a whole lot more than a USB memory stick (except that as a backup it’s more secure). Again, could be useful down the line of iPhone evolution, but not really yet.

5) Backup. While off-site backup is always a jolly good idea, .Mac PIM sync would accomplish this for PIM data, and I don’t really see Apple storing millions of users 16/32 GB media libraries anytime soon .Mac backup is currently far to slow for this anyway.

So I can see real potential in migrating at least some of the current .Mac to the iPhone. I do think though that there may be one major hurdle to this. Thus far, Apple seem to have been quite assiduous in not playing favourites when it comes to Mac iPhone users and Windows iPhone users, possibly because they don’t want the iPhone to be seen as ‘better if you have a Mac’, ‘not so good with Windows’. It wouldn’t be insurmountable of course to give something equivalent to the .Mac services to those using an iPhone with Windows, but it would certainly involve more work.

Additionally, the iDisk access would be great, but the iPhone really need the .mac PIM remote syncing. I really want to be able to sync my work PIMs with Activesync and my personal PIMs with .mac remotely since I use multiple macs and my wife and I share personal PIMs. I also think it would be a bit embarrassing for Apple to have the only push option be via a Microsoft product.