I received an email message this morning about Yahoo!’s Web Beacon which shares Yahoo! cookie information among Yahoo! sites and “certain partners”. Sounds like this lets Yahoo! et al share information about surfing and email opening patterns pretty widely. All for my convenience, of course. (See their privacy policy, too.)
They allow a per browser opt-out. If you opt out, just be careful NOT to click on the prominent “Cancel Opt-Out” button on the acknowledgment page.