I've had problems with iCal sync ever since I upgraded to Leopard. After looking at Console errors that said there were conflicts (multiple definitions of cryptic events) I finally noticed that (on one machine) I had two different repositories for calendar data:
~/Library/Calendars (proper location I think)
~/Library/Application Support/iCal/Sources (holdover from Tiger?)

I got rid of the App Support/iCal folder completely, since it did not exist on my other Leopard machines. I also deleted all of the preferences (there were 3 different files, though only the basic com.apple.iCal.plist file was recreated by iCal). After synchronization, it seems like all events are showing up. Now to upgrade to 10.5.2 to see whether I lose entire calendars, as some have reported to MacFixit.

I think the problem originates with the Migration Assistant not getting rid of enough stuff after importing and leading to conflicts later on.