The Collaborative International Dictionary of English v.0.48:

cloud nine \cloud" nine"\, cloud-nine \cloud"-nine"\n.
   a state of extreme or perfect happiness; used especially in
   the phrase on cloud nine.

   Syn: bliss, blissfulness, seventh-heaven.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

cloud nine
    n 1: a state of extreme happiness [syn: bliss, blissfulness,
         cloud nine, seventh heaven, walking on air]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "cloud nine":
   acme, apex, apogee, beatification, beatitude, bewitchment,
   blessedness, bliss, blissfulness, brow, cap, cheer, cheerfulness,
   climax, crest, crown, culmen, culmination, delectation, delight,
   ecstasy, ecstatics, edge, elation, enchantment, exaltation,
   exhilaration, extreme limit, extremity, exuberance, felicity,
   gaiety, gladness, glee, happiness, heaven, heavens, height,
   high noon, high spirits, highest pitch, highest point,
   intoxication, joy, joyance, joyfulness, limit, maximum, meridian,
   mountaintop, ne plus ultra, no place higher, noon, overhappiness,
   overjoyfulness, paradise, peak, pinnacle, pitch, point, pole,
   rapture, ravishment, ridge, seventh heaven, sky, spire, summit,
   sunshine, tip, tip-top, top, transport, unalloyed happiness,
   upmost, upper extremity, uppermost, utmost, vertex, very top,
   zenith