Khaled developed a text rendering engine for his needs, but quite honestly I don't find it very strong. I would personnally see a lot of new functionality (which I see in other IRC clients, or other communication toold in general) associated with a better text rendering engine.
Well, my point is that Khaled mut have found a hard time playing with his code to figure out how to render italic, and finally found that it was harder to implement than to say. Basically, rendering underscored or bold text is easy, each character still has the same position/width. Now when it comes to italic it's different. Look at some italic text, look in particular to the last letter... as you see, letter are slightly larger then they would if they wasn't itlicized. If you think of it from a programer point of view, you realize that you need some different method of calculation where each character should be drawn.
Khaled must have realized there was enough method of emphazizing text so that it wasn't much useful to waste time to implement italic.
