Windows or Mac? The OS doesn't matter in the ...

In the future you won't be concerned with wheter buying a mac or a pc. All your applications will run on pc or mac.

Michael Mace at Mobile Opportunity asks the question, why Apple ported their browser Safari to Windows. Answer:

Apple realizes that in the long term, the development platform that matters is not the OS on the hardware, but the software layer that the web apps run on

Such a software-layer is called "platform".
Michael already wrote about this topic in another blog-entry:

The distinction between platform and OS is important because the underlying operating system plumbing doesn't generate much value. It's the platform that users and developers are loyal to. If you separate the platform from the underlying OS, the user interface and applications can run on any OS. The OS itself becomes just commodity technology.

There is a lot of development in this area. Parallels Coherence brings Windows-Applications directly on the Macintosh-Desktop, like if they were Mac-Applications! I also had a little conversation about that topic on Tim Bruystens Weblog (german).
Update: Another program for virtualization of Windows on Mac: VMware Fusion
Here is a Demonstration-Video of Parallels Coherence: