macromedia flash sitedesign: I've seen a few sites that implement a sliding background image in horizontal
menus, that "catches" up to the cursor as you move left or right across the
menu, then hovers behind the cursor when it's static.

Anyone give me a clue as to how this is implemented or point me at a tutorial ?

cheers

Try these tutorials:
http://www.flash-creations.com/notes/dynamic_slidingviewer.php