And how the hell would it send Microsoft back into court? It doesn't nullify alternate APIs, it just means that the GUI is powered by DirectX
Lesser things have put them in court. Like when Netscape complained that IE was being distributed with Windows as if the user had no choice but to use IE - and that they did not have to pay for IE separately which gave IE an unfair advantage.

It didn't matter if their move nullified Netscape or not, because Netscape still existed and was being sold...but Netscape claimed it gave Microsoft an unfair advantage over their product.

I think legitimate legal claims could be made against MS if they choose to develop the GUI solely for DirectX 9.0. This would give them an unfair advantage over OpenGL obviously and you and I both know that when they move to DirectX 9.0 for the GUI, everything and anything about the GUI will be so geared towards DirectX 9.0 that GL won't stand a chance as far as desktop GUIs go.

So are we going to then have a choice between using OpenGL as our GUI or DirectX? Perhaps?

I think it spells trouble if they approach this issue as carelessly as they have similar types of issues in the past. I'm sure that they are thinking of this as we speak...so that they do not end up in another legal battle.

DirectX 9.0 is pure Microsoft and so is Windows. IE was pure Microsoft and so was Windows. What's the difference between these two issues. Not much I'm afraid.