Reuters had an interesting article on Chinese students gaming the GRE by setting up networks to share questions. Basically those who take the test post the questions online. Blogs and SEO ensures those seeking the questions can find them. Because ETS takes forever to ensure each question properly measures what it should, the questions are acquired faster than replaced.
Educational Testing Services places physical security on the tests to ensure the questions are not leaked by people acquiring copies of the test. Unfortunately, memorization of the questions is difficult to defeat this way.
Grade Point Average and tests like the GRE are common admission requirements to a graduate program. A high score becomes an obsession to students looking to attend their chosen program. The desperate seek any edge. Some people hire tutors or educational services who help learn how to take the tests. Bookstores carry study guides. Plenty of web sites offer advice.
The difference between legitimate assistance is the questions are not the real one. Studying the actual questions is crossing the line.
We see the same cheating behavior in other high stakes testing. The testing companies are have done such a great job making their tests the metric that a high score becomes so important people must have a good one. Therefore, defending the validity of the tests requires them to stay one step ahead of cheaters. Guess that is price of attaining the dream for a testing company.