The tab 'Design' doesn't appears in editor, disabling the form editor.
This only occurs in vb files because it's not necessary write “partial” in the one of partial classes.
The Visual Studio 2005, doesn't place 'partial' in the principal partial class.
The SharpDevelop needs to parse vb files looking for partial classes that don't have 'partial' in class declaration scope.
Example:
The VStudio creates:
'Public Class Form1' and 'Partial Class Form1'
The SharpDevelop needs:
'Public Partial Class Form1' and 'Partial Class Form1'