iSync version 3.0 is included with Leopard (in the Applications folder). Delete any 2.x or earlier versions of iSync. You can select iSync's icon in the Finder, then choose Get Info from the File menu to see which version it is.
Additionally, if you use a Dock icon for iSync that points to the earlier version, you may need to drag it off the Dock to remove it. Simply drag the Leopard version of iSync from the Applications folder to the Dock.
To avoid issues such as this, remember to always keep iSync and other Apple-installed applications in the Applications folder (dragging an application's icon to the Dock does not move the application).