After updating Leopard to version 10.5.2 using Software Update, iSync stopped working – it flashed in the dock, but never opened 
Luckily i found this dirty but awful simple trick to get it back to working:
- open Applications folder
- drag iSync app to your desktop
- open iSync from desktop -> it works!
- drag iSync from your desktop back to your Applications folder -> iSync still works and everything is back to normal
http://discussions.apple.com/message.jspa?messageID=6587458#6587458