TransparencyKey unsupported when using double buffer.

One of the easiest way to create transparent forms in Windows is using the Properties TrasparencyKey and BackColor of a WindowForm but after some hours struggling with strange behaiviors I discovered that TransparencyKey property is unsupported when enabling DoubleBuffer.

I can't beleive Microsoft couldn't mention it in ther official API:

http://msdn.microsoft.com/es-es/library/system.windows.forms.form.transparencykey%28VS.80%29.aspx