I set up Eclipse using the instructions posted on the (now old) WPILib site, here:
http://users.wpi.edu/~bamiller/WPILib/
That setup uses Mingw, but the best parts are the two plugins that Brad provides: one for error parsing, and one that provides an mcc18 toolchain that lets you use a managed make project.
Also, you don't have to use WPILib to use that setup. I also was able to compile and run Kevin Watson's code with only a minimum of changes (basically just leave out the WPILib headers and libraries).
Overall, it is the simplest path I have found to getting Eclipse working with the mcc18 compiler.