Although Yahoo claims that "No personally identifiable information about you is shared with partners from this research", many may wish to opt-out of the web tracking process. However, opt-out is browser-based. Hence if you use multiple browsers, you will need to opt-out separately from each one. And when you do opt-out, the page that is rendered may be slightly confusing to some users. Once you've clicked the link, you have opted-out. Do not click the grey button that says "Cancel Opt-Out". Instead, either click the back button on the browser, or simply close the window. The direct opt-out link is http://pclick.yahoo.com/p?optout.
This is not the first time Yahoo's marketing tactics have come under fire. In April 2002, Yahoo automatically signed their users up for spam, junk mail, and telemarketing. To counter it, Yahoo users had to login to their account and opt-out of the arrangement. (For more on this, see Wired News: Yahoo's 'Opt-Out' Angers Users)
Update: Since the original article was written (May 2005), Yahoo has changed the location of their Web beacon opt out page. The new page requires that you allow a cookie from Yahoo, which is counter-intuitive to what most folks consider opting out. If you are using Firefox with the NoScript addon (advisable for security purposes), you can more easily block Yahoo web beacons by doing the following:
1. Open the Firefox browser and select Tools | Options
2. In the Privacy section, under Private Data, make sure "Always clear my private data when I close Firefox" is selected and then select Settings
3. the Clear Private Data selection will open. Make sure each of the options has been selected (each should have a checkmark) and then click OK.
4. Click the Clear Now button to remove all stored cookies, web beacons, etc.
The NoScript addon in Firefox will prevent scripts from Yahoo and any other site unless you specifically allow them. And any cookies/beacons that are downloaded when you access a site will be deleted when you close Firefox.