I am curious, too, about the ability for the IM feature to connect to Yahoo!. It's covered in the Zimbra user manuals and many other places, but I've never had the option.
So, dalmate, since you are receiving an error, does that mean you actually have the option to add a Yahoo! buddy? Or are you just entering a Yahoo! e-mail address and crossing your fingers?
By the way, I am using 6.0 RC1 now, but the problem is/was the same for me when I was using 5.0
