Mine is still down (since yesterday morning), as is my wife's. Some people I know w/iphone+yahoo mail are unaffected.
Here's some background info and a response from Yahoo acknowledging some sort of problem on their end...
I've used my paid yahoo mail account inside the native iphone mail app (via the mail account settings) for a very long time and have not changed my password. Yesterday morning (June 27th), and continuing, I can't get yahoo mail to work from the native iphone mail app, complains about a password being wrong, or about the server rejecting my password (apple.imap.mail.yahoo.com on port 993), but my mail does work from a web browser when I log in using the same info as what is on my phone's settings (frustrating!).
Here's the response I got from yahoo customer care (they acknowledge a problem but haven't fixed it yet)...
Hello ,
Thank you for contacting Yahoo! Mail.
I understand you are receiving the "Invalid ID and password" error while accessing your Yahoo! Mail account using mobile device. I apologize for the inconvenience this has caused you. Let me address this for you.
The following Yahoo! Mail Help article should be helpful in addressing your issue. Please use the link below to review the article.
Yahoo! Mail users see "Invalid ID and password" errors when using mobile device
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http://help.yahoo.com/kb/index?page=content&id=SLN7959&actp=support&locale=en_US &y=PROD_MAIL_ML&i=+6Q0pgwl9BwsA51AEAt2zg==
I hope this addresses your issue. If you have any further questions or concerns about using Yahoo! Mail, please feel free to contact us at any time.
Thank you again for contacting Yahoo! Mail.
Regards,
Anglina
Yahoo! Customer Care