Yahoo! wasn't happy with Microsoft blaming their email systems for the Windows Phone 7 "phantom data" bug; the company pulled out a long list of rival OSes and suggested since none of them issues it was actually Microsoft's flaky IMAP support to blame. Unfortunately it seems they can't shake responsibility quite that easily; Within Windows' Rafael - who actually figured out the original Yahoo! issue hours before Microsoft announced it - has discovered that a similar problem occurs on the iPhone, too.
He looked at how Yahoo! mail servers handled iOS requests, and discovered a whole lot of extraneous information being fired back without actually being asked for. One suggestion is that, since iOS only fetches info on 50 messages at a time - rather than 200 - the unnecessary data use isn't so obvious, but it's still there. Android seems to be immune to the problem, however.
Microsoft has its collective hands full with the whole Bing/Google argument which is still ongoing, so they might be too busy to take Yahoo! to task on their server behavior. Still, both companies say they're working together to address the WP7 IMAP issue, so hopefully this extra wrinkle will get ironed out in the process.