The Collaborative International Dictionary of English v.0.48:

Heedful \Heed"ful\, a.
   Full of heed; regarding with care; cautious; circumspect;
   attentive; vigilant. --Shak. -- Heed"ful*ly, adv. --
   Heed"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heedful
    adj 1: taking heed; giving close and thoughtful attention;
           "heedful of the warnings"; "so heedful a writer";
           "heedful of what they were doing" [syn: heedful,
           attentive, thoughtful, paying attention] [ant:
           heedless, unheeding]
    2: cautiously attentive; "careful of her feelings"; "heedful of
       his father's advice" [syn: careful, heedful]
    3: giving attention [syn: advertent, heedful]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "heedful":
   accommodating, accommodative, advertent, agog, agreeable, alert,
   all ears, all eyes, assiduous, attentive, aware, canny, careful,
   cautious, circumspect, complaisant, concentrated, conscientious,
   conscionable, conscious, considerate, curious, deliberate,
   delicate, diligent, discreet, earnest, exact, finical, finicking,
   finicky, fussy, gingerly, guarded, helpful, hesitant, indulgent,
   intense, intent, intentive, judicious, leaving out nothing,
   lenient, loving, meticulous, mindful, mindful of others, nice,
   niggling, noncommittal, obliging, observant, observing, on guard,
   on the ball, on the job, open-eared, open-eyed, openmouthed,
   overlooking no possibility, painstaking, pawky, politic, prudent,
   punctilious, punctual, regardful, safe, scrupulous, slow to act,
   solicitous, tactful, tender, tentative, thorough, thoughtful,
   tolerant, unadventurous, uncommunicative, undaring, unenterprising,
   unprecipitate, watchful