Everyone who knows about Mix '07 
most likely knows that we're expecting a big announcement. I have to admit that I don't know what it is, but I have a couple of guesses. If I had to pick one, I'd have to say it'll be the announcement that the Expression tools will be available on MSDN. This has been a complaint I've heard from countless sources and, I honestly don't think Microsoft will drive too much adoption without this. I understand Microsoft's logic behind not putting the tools on MSDN, but I think it makes more sense to put it there.
I will say some of the talk I've heard leads me to believe my guess isn't big enough of an announcement. Trying to think of what else might be on the horizon is kind of hard, tho, seeing how open Microsoft is. Perhaps the biggest announcement I could see would be the release of Visual Studio codename "Orcas". In the same breath, I don't see this being released at a mixed developer/designer event; especially considering this event has a lot of designer-friendly content. With that, I'm left with the announcement of WPF/e and/or a friendly WPF/e design/development tool. If that's it, all I can say is I hope they come up with a nice, "cool" product name. I wish they'd just fall back on the codename, Sparkle. Beyond these ideas, I can come up with a few small possibilities, but nothing that is too big of a deal. I ultimately decided to skip out on this conference, so I won't be there to get the announcement, but I'm excited to find out what it is.