Courtesy of Engadget, here's Yahoo's statement on the subject:
"Yahoo! Mail is widely available on tens of millions of mobile phones, including those running on Apple iOS, Android, Nokia Symbian, and RIM. The issue on the Windows Phones is specific to how Microsoft chose to implement IMAP for Yahoo! Mail and does not impact Yahoo! Mail on these other mobile devices. Yahoo! has offered to provide Microsoft a near-term solution for the implementation they chose, and is encouraging Microsoft to change to a standard way of integrating with Yahoo! Mail, which would result in a permanent fix."
Microsoft has a long history of not taking POP/IMAP seriously, so I'm inclined to believe Yahoo in this instance. Microsoft has always been fixated on great Exchange support, to a fault, so this IMAP issue with Yahoo doesn't surprise me at all. Prior to Windows Phone 7, IMAP performance was horrific - my iPod Touch could poll my personal email, download messages, and be done before my HTC HD2 was done the initial check for new messages. Windows Phone 7 seems to be quite a bit better in that regard - checking IMAP email is quite quick - but it wouldn't surprise me that Microsoft perhaps took some liberties with how their implementation of IMAP works.
Did any Windows Phone Thoughts readers get hit by this bug? What did it do to your data consumption?