On the 8800, every time I plug it in, it asks me if I want to use it as a mass media storage device (or words to that effect). I *think* this happened on the 8100 I had as well, but I'm not sure. There may be a setting for this somewhere in the options as well, so check there.
FWIW, take the MissingSync survey and let them know what you want in the next Mac Blackberry sync product: >Survey Here<
IMHO, two crucial gaps exist: the ability to do BB OS upgrading and to sync password with SplashID.