Be sure to run iSync on all your devices before running the Tiger upgrade. It likely won't be the end of the world if you don't, but you'll be safer, i think. Tiger installs a new version of iSync, on which you should re-enter preferences on a per-device basis. iSync will be doing a "first sync" and offer you the option to merge or override what's on your devices with what's on your computer. If you merge, it says you're likely to have duplicate data, and that's because this version of iSync has no knowledge of records imported on past versions of iSync.
Basically, by the time you run the Tiger upgrade, you want to make sure that whatever lives on your computer matches data that lives on your various devices.