Visual Studio 2005 Beta 2 has a bug that breaks debugging for WinForm applications that host the Visio 2003 Drawing Control. The problem is caused by the new VSHost process (more information can be found here http://blogs.msdn.com/dtemp/archive/2004/08/17/215764.aspx). It is not possible to disable this hosting process in Whidbey Beta 1; however Beta 2 provides the option to turn it off.
Before you debug a project that hosts the Visio Drawing Control, go into the project’s settings, select the Debug tab, and de-select “Enable the Visual Studio hosting process.” Here's a screenshot for the setting.
Once you do that, you should be able to use the Visio drawing control in your VS 2005 winform.