Go to your Account Information screen (for each and every ID you have) and about mid screen you will see "Edit Your Marketing Preferences" link. Click on it and set them back to the way you want them, otherwise get ready for *LOTS* of advertising spam type emails from Yahoo's advertisers. Note also at the bottom, that you will be marked YES for 'By US Mail' and 'By Phone' as well."
In additional Yahoo News, smagruder writes: "Starting today, I noticed that Yahoo! stopped forwarding my mail and when I go to setup/change the POP Access/Forwarding settings, they display a page for me to give them money to get my mail forwarding back. The issue: In their recent widely distributed press release, Yahoo! said that this all would start on April 24, NOT March 28!"
Update: 03/29 20:24 GMT by J : Yes, of course Yahoo is a TrustE customer. For a small fee, TrustE certifies: "You can edit your Yahoo! Account Information, including your marketing preferences, at any time." Isn't that great? I can edit my marketing preferences that I had no reason to know existed! Thanks, TrustE!
Update: 04/07 11:54 GMT by J : Nine days later, Yahoo notified me that these preferences existed:
From: Yahoo! <email@example.com>
In order to keep you up to date about our many new products
and services and how they might be of use to you, we have
created a new Marketing Preferences page
within the Account Information area. It is designed to make
it easier for you to manage the marketing communications
you receive from Yahoo! and ensure you get the latest
relevant information to meet your needs. We have reset your
marketing preferences and, unless you decide to change
these preferences, you may begin receiving marketing messages
from Yahoo! about ways to enhance your Yahoo! experience,
including special offers and new features. Your new marketing
preferences will not take effect until 60 days after the date
of this mailing so you have plenty of time to decide what you
want to receive and what you don't. To change your
preferences, go to the Marketing Preferences page.