From WordNet (r) 2.1 (2005) (link)

  flabbergasted
      adj 1: as if struck dumb with astonishment and surprise; "a
             circle of policement stood dumbfounded by her denial of
             having seen the accident"; "the flabbergasted aldermen
             were speechless"; "was thunderstruck by the news of his
             promotion" [syn: dumbfounded, dumfounded,
             flabbergasted, stupefied, thunderstruck,
             dumbstruck, dumbstricken]

From Collins COBUILD Advanced Learner's English Dictionary (link)

  flabbergasted
    If you say that you are flabbergasted, you are emphasizing that you are extremely
    surprised.
    Everybody was flabbergasted when I announced I was going to emigrate to Australia.
   ADJ: usu v-link ADJ, oft ADJ by n, ADJ to-inf [emphasis]
  
  
  

From Soule's Dictionary of English Synonyms (link)

  flabbergasted
  a.
   (Colloq. and humorous.) Confounded, confused, abashed, ashamed,
   disconcerted, nonplussed, dumfounded (colloq.), taken aback, thrown off
   one's balance.
  
  
  

From Moby Thesaurus II by Grady Ward, 1.0 (link)

  49 Moby Thesaurus words for "flabbergasted":
     agape, aghast, agog, all agog, amazed, astonished, astounded,
     at gaze, awed, awestruck, beguiled, bewildered, bewitched,
     breathless, captivated, confounded, dumbfounded, dumbstruck,
     enchanted, enraptured, enravished, enthralled, entranced,
     fascinated, gaping, gauping, gazing, hypnotized, in awe, in awe of,
     lost in wonder, marveling, mesmerized, open-eyed, openmouthed,
     overwhelmed, popeyed, puzzled, rapt in wonder, spellbound,
     staggered, staring, stupefied, surprised, thunderstruck,
     under a charm, wide-eyed, wonder-struck, wondering