Not sure how I missed this one... But, I had been looking for a good, FREE, application for my Nokia E61 to do Instant Messaging on Yahoo! Messenger. And, I found one, and much more, in Fring.
From GigaOm on 10 Apps To Have For Your Symbian S60 Phone:
Fring — A multi-protocol IM client that currently supports AIM, Skype, Yahoo, MSN, Google Talk and Twitter, and features a SIP client. Fring seamlessly uses your data connection to allow you to IM or call contacts if they are on Google Talk or Skype. Fring also recently launched a feature that allows you to send files over Wi-Fi and 3G connection to your contacts. Very cool.
I just need the Yahoo IM connection, but I also tried Fring through my Skype account calling a landline, and it worked just fine (voice quality was about what you would expect - not great, but certainly understandable).