[GPS]

 proc flash.listbox.selection {listWin i} {
  set oldFg [$listWin cget -selectforeground]
  set oldBg [$listWin cget -selectbackground]
        
  $listWin config -selectforeground $oldBg
  $listWin config -selectbackground $oldFg
  incr i
   if {$i > 3} {
    return
   }
  after 60 [list flash.listbox.selection $listWin $i]
 }

Usage:
 pack [listbox .l]
 .l insert end Tcl and Tk are fun to play with
 bind .l <<ListboxSelect>> [list flash.listbox.selection .l 0]