Isn't that the synch feature? It checks what is on the playlist and deletes what isn't?
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This page details the sync system
http://docs.info.apple.com/article.html?artnum=60940
but
http://www.lordpercy.com/ephpod_ipod_sync.htm is a sync tool that looks interesting and a bit more flexible.
Not an IPod user myself so not sure, think Stu C may have more idea to be honest.