Cisco has recently released a new version of CCNA Security exam. The old exam 640-544 will no longer be available after November 30th 2015. The exam code is 210-260 (oh..its becoming so difficult to remember these exam codes!).
The new CCNA Sec exam is considered much tougher compared with its predecessor that several scenario based questions have been added now. The candidates need to have some hands-on experience in configuring IOS networks using ASDM GUI interface.
For more details, checkout the Cisco page here: http://www.cisco.com/web/learning/certifications/associate/ccna_security/index.html
If you are interested in knowing what are the differences between the old and new objectives, check this out:http://certforumz.com/index.php/topic,1269.0.html
Those already in final stages of preparation, it is recommended to take the exam before the last date (30th Nov.2015), unless you have more time and resources on hand for the new exam!
Update: Please check out CertExams.com CCNA Security Practice Exam
Simulator with most relevant practice questions and conforming to the latest exam topics.