How and what are you syncing?
A mobile phone using
iSync?
Or with MobileMe and other Macs/iPhone/iPod touch using
MobileMe Sync?
Your signature says you're still using Mac OS X 10.4.4 - you should really upgrade to at least 10.4.11 for best syncing results.
Have you tried resetting SyncServices by clicking *Reset Sync History...* in
iSync preferences.
Or if you're using 10.5.x as described here:
http://support.apple.com/kb/TS1627