Guessing, something we all do, every day, in nearly every aspect of our lives, has long been discouraged on College Board tests such as the SAT and AP. In fact, students were warned against random guessing in the most forceful way possible -- a wrong answer penalty.
On the other hand, the ACT, which has gained far more test-takers in recent years, doesn't have that penalty. During the summer, the College Board announced there would be a change in how AP tests are scored. To date, AP scores have been scored this way: total amount of correct answers minus a fraction for incorrect answers.
As the College Board states, "If you have SOME knowledge of the question, and can eliminate one or more answer choices, informed guessing from among the remaining choices is usually to your advantage."
The new grading feature will apply to AP tests taken in May 2011. Don't start holding your breath quite yet for a similar change on the SATs.