I'm having this problem with version 6.1.  I'm using Visual Studio 2008 vb.  The control will render fine in design view the first time I place it, but if I close the file and re-load it the control will not render. 

The page will run fine, but I have to remove and re-insert the control each time I re-load the file in Studio in order to work in design view.  Not sure if it makes a difference, but I have not purchased the product yet and am running the evaluation key.