<p>How hard would it be to use GCC instead of VC++ from within Visual Studio 2008? Obviously, some of the keywords won't match, and some may not get syntax highlighting (unless you made a new language service).</p>

<p>Is this what a 'makefile project' is for, pretty much?</p>

There is no GCC plugin for Visual Studio as the one for example for Intel C++, which allows Intel's compiler to be used as a drop-in replacement for Visual C++.
You can use Makefile projects which allows you to use any external tool you like (make, SCons, jam, whatever) to do your build. If you don't like Makefiles, you may want to look at
CMake
which allows you to generate Makefiles or Visual Studio projects from a much simpler description. This also means that you're not bound to Makefiles or Visual Studio projects, but can switch between them as you like.