If memory serves, the way it works is you buy the actual seats from the Autodesk website, and the "voucher" you get is good forever.
Then, once you have purchased your seats, you have to schedule the actual exam with an Autodesk Authorized Training Center in your area, and they will actually administer the test.
I recommend you take the class that Matt Murphy gives on the Certifications at AU2008 -- no guarantees, but last year the people who attended that class got a discount on the price of the actual exam (including discounts on study materials), on top of a little insight into how the tests are formatted.
I would recommend also that if you take the examination, take AutoCAD 2007 first, then
recertify on 2008 or 2009. That's the advice I was given, on account of the formatting of the actual test changed with 2008.
Details:
http://usa.autodesk.com/adsk/servlet...12&id=10095428
Hope this helps!