This is odd -- in the process of getting iSync set up on my Nokia 3650, OS X asked me to install the "Apple iSync Agent" on my phone. The installer actually sends the .sis installation file to the phone over Bluetooth, which leaves you with a new application on your phone:
This application must be running when you want to sync, otherwise iSync on your Mac won't recognize the phone.
This is odd for a number of reasons, not the least of which is that in previous versions of OS X, iSync used to talk to the 3650 with nothing more than Nokia's built-in SyncML dÃ¦mon running. Why Apple chose to write their own background agent, I can't answer. I suppose there are a number of Symbian series 60 phones out there, but wouldn't they all pretty much support os-level SyncML? My 3-year-old phone does, at least. And in years past, the syncing was comprehensive enough to even transfer the user photos between Mac OS X's address book and the phone's own directory.
On the other hand, there are some known limitations in the 3650 as regards calendaring. As I discovered this summer while working at a place with a comprehensive Outlook/Exchange infrastructure, the 3650 doesn't handle meetings that repeat more than once per week. (Every Monday and Wednesday at 2pm, for example.) I'm not sure if this is a limitation in the calendar itself or the syncing layer, but regardless, you end up with half your meetings transferred. Very annoying. Apple may have wanted to write their own sync system (and write to the calendar data files directly?) to get around this.