Psst - don't tell anyone - but the March release of Expression Interactive Designer ("Sparkle") and Expression Graphics Designer ("Acrylic") are now available. These releases are compatible with the February CTP of the WinFX Runtime Components, just to confuse everyone :)
As a quick reminder, Expression Interactive Designer is the designer-orientated tool for building WPF user interfaces; Expression Graphics Designer is a vector and bitmap graphics tool that happens to offer a very nice XAML export function. (For completeness, we also have some extensions for building WinFX applications using Visual Studio, available from the MSDN WinFX site.)
One caveat that you might want to be aware of: the February CTP of the Visual Studio Extensions for WinFX shipped with a bug in how it handles markup compatibility namespaces. This bug manifests most quickly if you use the Expression Interactive Designer March CTP on the same XAML file. We've added a hotfix into the February CTP and have slipstreamed the fix into the download site. This will be available in the next day or two, and I'll add a further announcement when that happens. For now, don't expect seamless interoperability between the two tools.