Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Ever since the industry was shocked by Apple's announcement to partner with Intel for the x86 architecture, people have been talking about the potential threat that might pose to Linux in terms of overall growth. For one reason or another, many believe that Linux would have a difficult time thriving as the desktop operating system with the new partnership. One of the main theories behind this is the possibility of dual-booting Windows and Mac OS X on Apple's hardware. After all, with Windows and Mac OS X on the same system, you get the best of both worlds.
You get the stability and security you want with Mac OS X on a regular basis, and a wide list of software applications with global compatibility on Windows. Cost of upgrading both operating systems is a completely separate thing and whether you would want to run both Windows and Mac OS X on the same setup doesn't matter; at least the option is there if you so desire.
Let's get one thing clear. Apple did say that its x86 Macintosh computers would be able to run Windows seamlessly, however, you shouldn't expect Apple to support anything related to Windows. This means no legacy drivers or anything else that Windows might need to work on the new platform, but that's not a major concern. It's almost certain that a third-party company might jump onboard and deliver the needed drivers to run Windows on the same platform as Mac OS X. As mentioned earlier, you have the best of both worlds to take advantage of.
However, that doesn't mean it's the end of Linux. In fact, it shouldn't even threaten Linux by any means. Linux has more than a few things that go in its favor, at least for the time being. The idea of open-source software is an amazing one. The fact that Linux isn't much of a commercialized operating system, and you can accomplish day-to-day tasks without too many hassles is an advantage in itself. The idea of running a system that costs absolutely nothing on the software side is a powerful one, and Windows and Mac OS X would have a difficult time competing against that.