When I changed the ContentPlaceHolderID in the Master Page (and the content pages) and then tried to view and content pages in design mode I got the following error:
"The page has one or more <asp:Content> controls that do not correspond with <asp:ContentPlaceHolder> controls in the Master Page"
For some reason the Visual Studio 2008 web page designer mode does not like having XML shorthand notation of the <title /> tag in the <head> section in the master page.
So this causes the error:
Changing the <title /> to <title></title> fixes the problem: