We wanted to ensure that when using a particular form in AX2009 client no other forms/reports are accessible until the form is closed.
For this we need to make the form a modal form.
We have an example of this in the standard application which can be copied, look at: Classes\SysModalFormDisableAllWkspcs.
This is used to make the Organize favourites form in AX modal (the name of that one is SysFavoritesOrganizeFavorites).
The way we achieve it is by starting the form like this: