I've heard good thngs about
MySync, though it seems a bit dodgy to trust that kind of data to software that still calls itself "beta."
But in any event, you could always try it out and have a manual backup plan in place while you make sure that MySync plays nice.
I keep thinking of ditching .Mac (I'm sure my parents would learn a new e-mail addy for me eventually), and I imagine if I ever get 'round to it, I'll scoop up this App.