Wow, such friendly and helpful responses, NOT!
I may be biased, because I have the same problem, but I think vbrechbill's question is completely reasonable. If you paid closer attention to his question, you would realize that his problem is with yahoo mail, not a website, therefor the password is probably not stored in the Safari keychain, but somewhere by the mail app.
So the question becomes, "Is there a way to view the stored email passwords of the iPhone mail app?"
A simple solution involving only looking on the iPhone is preferable to trying to jump through password changing hoops given the constraints of vbrechbill's predicament. So, if you know the answer to this, it should be straight-forward, no need for unhelpful condescending remarks.