iChat is definitely going to be hugely important for Apple but so is letting all these features work seamlessly together with Windows users. The few options Apple has in this regard is making the iPhone Mac only or with Windows compatible apps or just bring osX to PC and be done with it.
If we look at all the devices we want from Apple, they all need tight integration with the OS. An Apple branded iPod, iPhone, iTV, iSmart, iCamera, etc.. will be on the market sooner or later, so Apple will make and support countless Windows Apps or be osX only. I strongly believe that opening up osX will be easier to do and has a higher long term potential.