Yes, it is more complex than to use the icon.
I found the icon code myself, but as for now, I have no idea where to look for if I would like to change to mouse over the subject (which behaviour is desired by our users because they are used to it). Anyway, it seems that the bubble mechanism is ready for that.
One thing more... do you find it reasonable to show the bubble icon only when a post has a non-empty body text? This condition is already checked somewhere in the code, to determine if the "no_text.png" icon should be shown.