AD in W2K8 is restartable as a service.
This can allow an offline defrag (though whether this is of much use is debatable). However, it does allow the ntds.dit file to be compacted so you will likely see a decrease in the size of the file.
AD does do a regular (12 hours) online defrag as part of ongoing garbage collection process. This optimises data storage and disposes of tombstone objects, but won't decrease the size of the database.
Therefore the offline defrag can help to shrink the database and allow it to more easily be held in memory space and gain the performance gains that that entails.
See link for instructions.