Apple Support Document 300225: Photo Sync creates iPod photo Cache:
When using iTunes to sync photos to iPod photo, iTunes creates a folder called iPod photo Cache in the top level of the folder you selected for your photos. Picking another folder to sync does not erase the previous iPod photo Cache.
Depending on how many photos are being synced, the hard drive could fill up.
Drag the folder named iPod photo Cache to the Trash or Recycle Bin.
Sure enough, poking around in there I found cache files from 2005. so I moved some of that clutter over from my old PowerBook to the new MacBook Pro.
Juggling free space is always an issue for me, deleting this folder recovered almost 3 GB.
I noticed this folder while showing Disk Inventory X to a friend.