Periodically MobileMe users and people who have synchronized iPhones and iPods to their Macs have run into problems where items will not synchronize or improperly delete entries, duplicate them, or show other odd behavior. When this has happened, I usually suggest resetting the synchronized data on various devices, and then re-sync them all from one device, using it as a master source. If this fails to work, then resetting the SyncServices folder can help.
The process for doing these things is slightly different for various versions of OS X, and with the release of Snow Leopard the process is slightly different still (though mainly because of slight reorganization of settings). This Apple knowledge base document outlines the various ways to approach resetting the sync services in OS X, including Snow Leopard, so if you are having problems with synchronizing, then this may help.