I thought this might be of interest to those that use Firefox 4. Its important to note that sites are NOT obligated to stop tracking you if you check the box.
"Honoring this setting is voluntary — individual websites are not required to respect it. Websites that do honor this setting should automatically stop tracking your behavior without any further action from you. "
I'm not sure, I cannot say anyone can be sure - but would you really want to risk it? I would say assume they are tracking and act accordingly - that way you cannot be wrong