Well I was literally about to sit down and write about my disappointment in the November WinFX CTP's absense of a visual designer for XAML, when I saw Aaron Stebner's blog announcing the December CTP. Too bad I just recently got the November CTP installed! (it's installer kept complaining about a CAB that wasn't properly signed, finally got it installed by deselecting the docs).
The December CTP boasts a visual designer, “Cider”, for XAML applications. While I'm sure it won't be anything fancy like Sparkle promises, it'll be a very welcome preview.
Once I've had a chance to try it out I'll post more...