ZenTech:
Cloud computing will hopefully open up the door to os independant applications but that's a long time away. I have high hopes that someday users will be able to choose to buy either a mac or pc, run either windows or linux and be able to run the same applications across whatever platforms they choose to use. I truly believe this day will come, eventually.
With virtualization becoming increasingly accessible (and open sourced with projects like Virtualbox, kvm, and xen), we already find ourselves able to run Linux and Windows at the same time. I have an install of ubuntu 8.10 alpha running as a vitual machine on my wife's imac (running Leopard) and I have w2k running inside virtualbox when ncessary on my laptop (which runs ubuntu 8.04).
Virtualization for the desktop has literally exploded on the scene in the last few years with machines regularly having multiple cores, virtualization-ready processors, and 2-4GB of memory being an affordable option. The only thing you lose currently is the best graphics tricks of the virtualized OS (which usually contends with a virtual version of intel graphics). Even this may improve in time.
An option for the Linux folks who want to run windows program would be to run their programs with the wine libraries - a set of windows-compatible libraries that run windows programs natively in windows - without actual virtualization. Things run faster and even 3d graphics are available. Google's versions of picasa and google earth for linux are essentially their windows programs running on bundled wine libraries.