Can Mac OS X distribute applications/processes across multiple intel cores? Or does efficient usage of cores depend only on whether a given application is written to take advantage of them? I am deciding between a Mac Mini - Intel dual core i7 2.7Ghz vs. one with a quad core i7 2.0Ghz.
Any modern operating system, including OS X of course, will take advantage of multiple cores as best it can. Applications need to be multi-threaded to truly take advantage of this within themselves,  but even when an application is not able to take direct advantage of multiple cores, the OS will still use its resources efficiently by finding something else for the 'idle' cores to do.