Yesterday I had this strange problem in Visualstudio 2008.
From one day to another the designer stopped rendering my controls in all my contentpages.
I have two nested masterpages and then contentpages.
The masterpages was working fine in vs designer, but the content was showing a blank page.
After several hour I found that a small update in the master page caused the problem.
I always use <div> insted of <asp:panel>. The problem was that I replaced a <div> with <asp:panel>. When I returned to use <div> everywhere in the masterpage, the vs designer suddenly started to render all contentpages again.
I have no idea why. I thougt <asp:panel> was the same as a <div> ?
Hope this can help someone with the same problem.