I'm able to sync my P910i using iSync. Both iCal and AddressBook data is synchronized correctly as far as I can tell. I have had some errors reported on the phone, but I think that's because some of my third party apps (like PuTTY) aren't going in the background correctly.
However, iSync was not displaying an icon for the phone. After consulting Dr. Google and reading up, I was able to figure out that /System/Library/SyncServices/SymbianConduit.bundle/Contents/Resources/SupportedDevicesInfo is the file that maps the firmware rev to the iSync device type. When I search that file for P910i, I see the key is P910-1. However, there wasn't a corresponding tiff file in the Resources directory, so I copied P900.tiff to P910-1.tiff.
It works!
