ASP.NET vs

It seems I am missing something. But ever since .Net 4.0 came out I have been seeing reference to <%: %> syntax for ASP.NET. It seems like it is similar to <%= %> but clearly there is a reason for the new syntax. I tried google but searching <%: %> doesn’t seem to return anything. What does the <%: %> syntax do? Was it introduced in .Net 4.0 or 3.5? Can someone enlighten me?

System.MethodAccessException: Attempt by security transparent method to access security critical method fails on all applications

Hello and thanks in advance for the help,
I know that this question or similar questions have been posted, frequently with specific regards to MVC 3 applications. However, I am getting this error message whenever I try and launch any application from within visual web developer express 2010 using a .net 4.0 target framework.