The Collaborative International Dictionary of English v.0.48:

Conscientious \Con`sci*en"tious\, a. [Cf. F. consciencieux, LL.
   conscientiosus.]
   1. Influenced by conscience; governed by a strict regard to
      the dictates of conscience, or by the known or supposed
      rules of right and wrong; -- said of a person.
      [1913 Webster]

            The advice of wise and conscientious men.
                                                  --Prescott.
      [1913 Webster]

   2. Characterized by a regard to conscience; conformed to the
      dictates of conscience; -- said of actions.
      [1913 Webster]

            A holy and conscientious course.      --Abp.
                                                  Tillotson.

   Syn: Scrupulous; exact; faithful; just; upright.
        [1913 Webster]

WordNet (r) 3.0 (2006):

conscientious
    adj 1: characterized by extreme care and great effort;
           "conscientious application to the work at hand";
           "painstaking research"; "scrupulous attention to details"
           [syn: conscientious, painstaking, scrupulous]
    2: guided by or in accordance with conscience or sense of right
       and wrong; "a conscientious decision to speak out about
       injustice" [ant: unconscientious]

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "conscientious":
   accurate, active, attentive, careful, cautious, censorious, choicy,
   choosy, circumspect, close, compliant, conforming, conscionable,
   constant, correct, critical, deferential, delicate, demanding,
   detailed, devoted, devout, diligent, discreet, discriminating,
   discriminative, duteous, dutiful, ethical, exact, exacting,
   exigent, exquisite, fair, faithful, fastidious, fine, finical,
   finicking, finicky, fussy, heedful, high-minded, honest, honorable,
   incorruptible, just, loyal, meticulous, mindful, minute, moral,
   narrow, nice, obedient, observant, overconscientious,
   overscrupulous, painstaking, particular, perfectionistic, picky,
   politic, practicing, precise, precisianistic, precisionistic,
   priggish, principled, prudent, prudish, punctilious, punctual,
   puristic, puritanic, refined, regardful, religious, respectful,
   responsible, right, right-minded, righteous, rigid, rigorous,
   scrupulous, scrutinizing, selective, sensitive, serious, strict,
   subtle, tender-conscienced, thorough, true, upright, upstanding