WordNet (r) 3.0 (2006):

rubber stamp
    n 1: a stamp (usually made of rubber) for imprinting a mark or
         design by hand [syn: handstamp, rubber stamp]
    2: routine authorization of an action without questions

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "rubber stamp":
   John Hancock, OK, accept, acceptance, accredit, affirm, affirmance,
   affirmation, amen, approbation, approval, approve, authenticate,
   authentication, authorization, authorize, autograph, certification,
   certify, confirm, confirmation, cosign, countersign,
   countersignature, endorse, endorsement, give permission,
   give the go-ahead, give the imprimatur, give thumbs up, go-ahead,
   green light, imprimatur, initial, nod, notarization, notarize,
   okay, pass, pass on, pass upon, permission, permit, ratification,
   ratify, sanction, say amen to, seal, second, sigil, sign,
   sign and seal, signature, signet, stamp, stamp of approval,
   subscribe to, subscription, support, swear and affirm, swear to,
   the nod, undersign, underwrite, validate, validation, visa, vise,
   warrant