Foogazi: "The most obvious and important reason your parents should run Linux is the security the Linux operating system provides." - I don't think so folks. The most obvious choice is Macintosh, period. If your parents already have a Windows box and $600.00 they can score a Mac mini and hook it up to their existing monitor, keyboard, and mouse. Apple pays very close attention to the user experience, everything just works. When we bought our daughter a Mac laptop she popped it open and started working day one, without Dad's help. The only reason I knew it had arrived is she called for the wireless password. I've provided ZERO technical support in a year-and-a-half. Try that with a Linux box.
The open source camp should take a close look at the Macintosh user experience and clone that instead of the Windows experience. If people want Linux to be adopted by mom and pop it's going to have to work WITHOUT configuration when the box is started, no matter what the environment. That has been my experience with Mac.