Do you know about “Flash cookies” and consumer privacy? This Demystified blog has an excellent exposé… see also:
A pilot study of the use of ‘Flash cookies’ by popular websites.
We find that more than 50% of the sites in our sample are using flash cookies to store information about the user. Some are using it to ‘respawn’ or re-instantiate HTTP cookies deleted by the user. Flash cookies often share the same values as HTTP cookies, and are even used on government websites to assign unique values to users. Privacy policies rarely disclose the presence of Flash cookies, and user controls for effectuating privacy preferences are lacking.
http://billmullins.blogspot.com/2009/09/lso-flash-cookies-serious-attack-on.html
There is a major advantage for an advertiser to employ Flash cookies, not the least of which is; they are virtually unknown to the average user. Equally as important from an advertisers perspective is; they remain active on a system even after the user has cleared cookies and privacy settings.
To call this a deceptive practice would be a major understatement. Crooked, immoral, fraudulent, illegal, are just some of the words that come to mind.
There are tools to help you delete these unwanted sneaky cookies… for Firefox, use BetterPrivacy.