Same problem
I seem to be having the exact same problem. My iPhone 4 syncs the 2 gmail accounts I have on there fine. But, the old yahoo mail account I have just sits and spins. Sometimes, if it sits long enough, it might finally get a message today (10/19), but yesterday (10/18) it would just sit there forever and not retrieve any new messages. It worked fine last night over wifi, but it did not work over wifi this morning. I know yahoo was having trouble with attachments yesterday, I'm wondering if the problem is more widespread than just attachments. Like their servers aren't talking to mobile devices propperly. I just got iOS 5 last friday and it worked fine with yahoo for the first 3 days, so I don't think it's the new OS. I tried deleting the account, setting it up as an imap account instead of the yahoo! setup in the mail setting, and resetting the phone. None of this seemed to fix the issue. I deleted it again and set it back up through the mail settings as a yahoo! account. Still having trouble this morning. It randomly got my latest mail and made all the contents an attachment.