Open this file in Property List Editor, find the AutoSyncInterval key, and then set it to the number of minutes after which you want it to sync with the server. I have it set to 1 right now, and it syncs every minute which is good enough for me at the moment. Hopefully this will be of value to anyone who needs something more immediate than a 15-minute sync, and doesn't want to bother with Manual sync.
[robg adds: I was disappointed to read that the 15-minute sync from a Mac to MobileMe/iPhone is a feature, not a bug. This hint seems to work well to create much more of a true push from your local Mac to the cloud; I tested it with both 1-minute and 5-minute settings, and it seemed to work just fine, and my setting survived even after opening and working with the MobileMe System Preferences panel. Obviously, though, if you use the Synchronize pop-up to change the sync interval, your settings will be overridden.]

