T-Mobile just started selling the Motorola V330, and I decided to change out my 3650 for it. It turns out the V330 is a V551 in disguise, so a little copying and pasting got iSync to see it just fine. This is on Mac OS X 10.4, so your mileage may vary.
Edit /Applications -> iSync.app [Control-click and pick Show Package Contents] -> Contents -> PlugIns -> ApplePhoneConduit.syncdevice -> Contents -> PlugIns -> PhoneModelsSync.phoneplugin -> Contents -> Resources -> MetaClasses.plist.
Once in the file, add this text, preferably below the V300 section. After editing, iSynch synced up my Address Book and iCal just fine! I was impressed.

