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.