Yahoo! Rejects! Ad-blocking! Users!
A thread on the Adblock-plus forums (and picked-up by the Washington Post and Engadget) indicates that 90's era Web portal and free-mail provider Yahoo! has started rejecting users from accessing their Webmail-boxes when using popular ad-blocking software. Unlike other mail providers, including free-mail providers such as Yandex and Google, Yahoo! doesn't support retrieving mail via IMAP (presumably because nobody used IMAP in the 90s).
Engadget reports that this is currently only a test by Yahoo! on a segment of their users, and the ABP forums indicate switching to basic mode allows one into the inbox. Security experts often recommend using ad-blocking software when browsing the Web as ad-networks tend to be a common source of malware infections.