Actually I would like to bring some precisions to my first post.
My application is a MDIParent with a child form. The MDIParent contains the Treeview and the child contains the textbox. A sample can be downloaded here:
http://graphicsxp.free.fr/ValidatingSample.zip (click on New Document to open the child form)
This bug do NOT occur with VS2003, only with VS2005 where the MDIParent.ActiveControl is always the child form....

Can you help ?