I'm a firm believer in recovery runs following races or hard workouts.  I don't know if the benefits are physiological or psychological, and I'm not sure I really care.  I just know that I tend to feel much better the day after a recovery run whereas if I take the day off after a race I usually feel "flat" the next time I run.

Patrick - I prefer to do some other activity (bike/swim) on recovery days rather than run, because I don't really think there is a true recovery run.  As a triathlete, I find that running takes the greatest toll on the body, and any run isn't really recovery.  However, for someone who is solely focused on running, I can see where a "recovery run" or easy day would be beneficial in keeping the body active and recovering while not taxing it too much.  In swimming and biking, I will do recovery sessions at an embarassingly slow pace.  In my opinion, recovery involves something different from what you are recovering from, which is why I tend to use a different sport for recovery.