Allow conditional compile statements in XAML markup
Allow the XAML markup editor to allow conditional compile statements so that a developer could have #ifdef statements in their markup
Support conditional compilation inside Xaml to account for Silverlight/WPF differences in shared markup.
Understand the design-time tooling experience for this could be a lot of work.
James Abbott commented
This!! Lots of times when I'm debugging I'll use a standard resource dictionary, but when in Release Mode I want to use a cached (I'm sure you've heard of it) StaticResourceDictionary. This is just not possible as far as I can tell currently, I'm sure there are dozens of other uses for this though!