GMAC recently announced that test takers of the GMAT will have the option of deciding whether or not to keep their results, immediately upon previewing their unofficial scores, after the end of the examination.

• You will be given 2 minutes after the end of the test to decide whether to keep your score or not

• It is important to have decided on the range of scores you are prepared to keep, beforehand.

• It will be possible to reinstate a score you have cancelled, for a fee

See the full announcement:
http://www.mba.com/global/the-gmat-exam/gmat-exam-scoring/your-score-report/preview-your-gmat-score.aspx