It is common for PhD students in Computer Science to assist their advisors with reviewing conference papers, known as subreviewing. The students' reviews may or may not get turned in with the actual reviews but it is a great form of practice for the students.
I have seen some students list the conferences they have subreviewed for in the Service section of their CV. I thought this was a little strange since it isn't official.
Should a grad student list this on their CV? It would help add some content to a short CV and show that they have at least some experience reviewing. It may also help a student associate with a conference that they haven't had the opportunity to publish at, yet.