Prefer iPhone when synchronizing playlists between iTunes and iPhone
I'm unaware of any settings that this will work with iTunes/iPhone out of the box. But you could give iGadget X a try:


  ** iPad, iPod touch and iPhone Support **
  
  iGadget (Windows and Mac) now supports
  the iPad, iPod touch and iPhone.
  However, due to the limitation of the
  Notes feature on these devices, only
  the Music Transfer, Playlist Transfer
  and Music Listing functions are
  available when used with the iPad,
  iPod touch or iPhone.




PS: It's shareware ($19.99)