This new feature, which isn't limited to Windows Forms, allows a single class to extend across multiple files, isn't limited to Windows Forms but the designer-generated code is just screaming out for a solution like this.

Jay writes:

Winforms designed code and C# partial classes

In Whidbey, the WinForms designer takes advantage of a new C# language feature called “partial classes”.  This allows them to pull out the designer generated code into a separate file.  It has several advantages...