This is a rather interesting problem, but it's not specific to the iBook. I'd like to say it belongs in the Networking forum, but it may have more to do with Safari than anything else.
What you're saying is that, when you put the cursor over the send button, it does not react as if it were over an active link. What you may not have been aware of is that recently Yahoo! reworked their mail interface. It may not be compatible with your specific version of Safari for one reason or another.
For a start, you could try installing Firefox, a freely available browser based on the Mozilla engine, and found at
Mozilla. Since you'd be installing a NEW browser that is up to date, that would show whether or not the problem was your browser or something else. Once you install Firefox, try to send a Yahoo!-based email and see if it works. If so, then Safari is at fault and you should post a new problem about Safari in the
Applications forum. If not, it may be your network setup, and you should post in the
Networking forum.
Oh, and welcome to MacNN!