TryI'm wondering how to turn it on! Ha, this is great. Actually, having a rollover would be very annoying. But I just switched to Safari 3.1 on Windows from IE 7, and I find I miss knowing what the URL will be before you click on it. In IE 7, it appeared in the bottom left side of the browser. It helps to be suspicious of where links might be taking you, IMO.
Is there a way to turn off the URL appearing and only have the alt-text appear? I wonder why the OP sees the whole URL and I only see "Mac Forums"
To turn off Safari tooltips that show URL of links:
in Terminal, type:
defaults write com.apple.Safari WebKitShowsURLsInToolTips 0 (1 to turn on)