It may need to be tweaked, but probably does not. It doesn't change anything about the video card -- that's why it works in a wide variety of different macs using the same code. The problems mostly occur with iBooks that have video cards that don't support spanning at a basic level, I think.
Once it does get working, it should actually be better, since that's a substantially faster and more capable video card than the one in the previous iBooks. Although, I guess you could also argue that, having more capability, more stuff will be offloaded to that video card than the old one, meaning more video ram will be used up, meaning, that, for the typical usage profile with the new card, you might see more deterioration in performance with the second monitor plugged in than you see on the old iBooks. But it'll still be faster than the old iBooks. 🙂
Anyway, though, I would wait and see if I were you, too, though. 😱