Am stuck with this too, driving me nuts! Got Tiger on PM and now on iBook. iCal stuff is on the PM and i can sync my iPod on this, but when i try sync the iBook to get my calendar updated to be same as PM is doesnt work.
I have gone into the .mac prefs and tried to register both machines, but it just wont have it.
Do i need to delete some prefs somewhere - help!