I'm sorry to hear that. I have gotten good feedback from most Leopard
users. As far as I know the syncing in Leopard should work as well in
Leopard as in Tiger.
I think that the sync engine that in OS X that keeps track of the sync
state for all syncing programs have entered a confused state one or
more of your computers. If that is the case, you will probably
experience similar syncing problems with other programs.
One simple way to try to fix the problem is to do a backup from within
iCal and than restore it. This will reset the state of the calendar
and todo data in the sync engine.
Best regards,
Jacob Wallström
http://ghostparksoftware.com