Same problem here.
I was tidying my inbox in Yahoo by sending emails I have already read into their respected subfolders. Voila, they are gone from both the phone and the web.
This is very inconvenient as these emails were important. Called Apple Support and the genius told me to (surprise surprise) do a soft reset and restore my settings. After that didnt work, he told me it wasnt their fault hence its wasnt their problem. I told him it is in fact their problem because they have adopted yahoo's services onto their hardware. Guess I cant rely on the email program for now.
