The catch is that you must fail the test the first time
“Come Back” Promotion
Here’s how to redeem your Cisco “Come Back” Exam:Register for an exam at full price. If you fail the exam, you may schedule a free retake of the same exam by entering the promotion code: COMEBACK at the time of registration.
IMPORTANT NOTE: Both the full-price exam and free Come Back retake must be scheduled and taken between April 15, 2008 and October 15, 2008, to qualify for this special promotion. Free exams may only be scheduled via the web. More…
UPDATE: It looks like there is another catch to this
Quote from Cisco Certification Support:
The “Come Back” Promotion applies to candidates whose certifications have expired.
The first attempt must be paid, but the second is free. This promotion does not apply if you are currently certified.

I have recieved following reply from cisco certification support.
Dear Muhammad Omair,
Thank you for contacting the Certifications Support Center.
The “Come Back” Promotion applies to candidates whose certifications have expired.
The first attempt must be paid, but the second is free.
This promotion does not apply if you are currently certified.
Cisco is reaching out to two distinct audiences who may be hesitant about taking a Cisco exam.
The Second Chance promotion encourages those who have failed an exam in the last year to try again. If they take a full-priced exam now and fail, they can get a second chance in the form of a free retake. Details can be found at http://www.pearsonvue.com/cisco/secondchance.
The Come Back promotion is for anyone who has achieved Cisco certification in the past, but for whatever reason has let their certification credential lapse. If they take a full-priced exam now and fail, they can get a second chance in the form of a free retake. Details can be found at http://www.pearsonvue.com/cisco/comeback.
With each of these promotions, both exams must be scheduled and taken between April 15, 2008, and October 15, 2008 and regular retake rules apply. Currently, the free retakes can only be scheduled via the call center, although plans are underway to make web registration available soon. Candidates who are eligible for these promotions were invited to participate in an email blast sent by Cisco and they will receive a reminder midway through the promotion period.
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