I got both of my yahoo accounts working on my phone w/out a problem. My wifes phone on the other hand gave me fits. When in doubt, remember you're dealing with a computer. Shut itdown and try again. After that, it worked, just like it did on my phone.
For the record, those same settings work for ymail too. I found the trick on another forum. You have to set up a regular yahoo adress first to get Android to accept the servers addresses. Then after that address is working, you can go into the account settings and change the UN and PW to your ymail leaving the other settings as-is.