The Collaborative International Dictionary of English v.0.48:

I O U \I O U\, IOU \I.O.U\([imac]" [=o]" [=u]"). [phonetic, from
   I owe you.]
   A paper having on it these letters, with a sum named, and
   duly signed; -- in use in England as an acknowledgment of a
   debt, and taken as evidence thereof, but not amounting to a
   promissory note; a due bill. --Wharton. --Story.
   [1913 Webster]

WordNet (r) 3.0 (2006):

IOU
    n 1: an informal debt instrument; representing `I owe you'