I think we've seen a number of people problems such as this, but no solution. I seemed to have this issue with some items in iCal right after I upgraded to Leopard. When it went to sync w/ .Mac, it had conflicts with some events being off by a day. I don't sync OF with iCal so I don't think this is an issue w/ Leopard iCal and syncing.
I eventually cleared all the conflicts and reset .Mac syncing and I think my dates are ok now. I couldn't find much with a google search, but did find this one showing the issue with another app. See message #14 on the following page:
http://forum.plaxo.com/showthread.php?t=4316&page=2