On December 5th (revision 3655), we removed the WPF designer from SharpDevelop 3.0. The decision to remove this feature had its origins in an internal discussion between Daniel, Ivan and myself after watching the PDC 2008 session Microsoft .NET Framework: Declarative Programming Using XAML. There, System.Xaml.dll was announced - a parser for the full Xaml standard.
Back then, a preview was announced for November, but that has been pushed back - please see the System.XAML CTP blog post by Rob Relyea. Therefore we had to take the decision whether to use System.Xaml.dll without doing a prototype, but we opted for saving ourselves the chore of having to keep abreast with future XAML changes by sticking with a standard parser instead of having to maintain our own.
Because System.Xaml.dll is part of .NET Framework 4.0, we had to remove the WPF designer feature from SharpDevelop 3.0.