I was able to successfully setup my "free" yahoo email account on my Eris. Although, the only problem is that the system doesn't update on the website, in other words, whenever you read Yahoo emails on your phone, they still show as not read on the website. I followed these steps exactly:
Go to Mail icon. If this is the first time you've setup any mail service, you will be prompt to enter in a username and password. Enter in your Yahoo email address and password
Click on the Manual Setup at the bottom left corner
In the incoming server setting, entering as followed
username:
xxx@yahoo.com (enter in your complete yahoo email address)
password: enter in your yahoo password account
IMAP Server: imap.mail.yahoo.com
Port: 143
Security Type: None
IMAP Path Prefix: leave blank / Optional
Press next to continue with SMTP Server. Once on that screen, enter in these settings
SMTP: smtp.mobile.mail.yahoo.com
Port: 587
Security Type: None
Check "required Sign-In"
Username:
xxx@yahoo.com
Password: your email password
Now click through next and fill out the name, etc. You are done. You now have IMAP Yahoo Mail Account setup!