I did see the thread about an iPhone app, which was not too popular, but I thought this idea was unique enough that it should be posted.
I'm currently contributing to all four SO sites. Particularly where Super User and Stack Overflow are concerned, I'm finding it difficult to keep up. Perhaps I need to choose which sites to contribute to in order to keep myself from going nuts, but I also think a desktop app would have a chance at better integrating the sites for those of us who use more than one concurrently.
Also, it might help bring into prominence some of the SO family features which are currently a little underused, like favorites, notifications (one e-mail a day?) &c.
I am not capable of building such an application myself, but I'd be interested to see what people think of this idea.