Basically, neither Visual Studio designer, nor Resharper seem to deal with the StaticResource markup extension when I use it "normally" - without the brackets.
<StaticResource ResourceKey="SomeKey"/>
It's not an issue of finding the resource, it chokes on the property name of 'ResourceKey'. So when designer loads, or if I use Resharper's solution analysis, my error windows are always cluttered with endless instances of "Cannot resolve symbol 'ResourceKey'"
I'm using Silverlight 4 with VS2010, but I had this problem with Silverlight 3 and '08 too. Recently reinstalled the whole stack for unrelated reasons, problem remains.
This isn't a showstopper, because everything works fine at runtime and in Blend (and I turn off VS xaml designer anyhow), but it is annoying, and most importantly, puzzling.
Don't know if it's related, but when I mouseover the "StaticResource" it tells me that the class is "ms.internal.metadata.exposedtypes.presentation.staticresourceextension" - but I was pretty sure that it lives in the System.Windows namespace.
Please, overflowers, shed some light on this mystery for me.