The survey house / think tank Gartner believes Unix is about to be replaced by Linux. I think it probably already has been replaced, with a few bastions of support left in companies that just don't want to upgrade. Gartner's point, here, though is that the applications specifically developed for Unix will wane after 2009. That makes sense. Most people don't make tires for Model A's anymore.
Linux is stable and mature enough (and similar enough to Unix) that it makes perfect sense to move from Unix boxes to Linux boxes.
