Windows 7 is coming later this year. Ubuntu is on 9.04 with the next version due out later this year. Mac has Snow Leopard coming out later this year. Then there is Red Hat, SUSE, and many other Linux flavors around. Has the day of the OS come to an end? The obvious answer is no, due to software that is written specifically for certain OSes. The answer is also yes because so much of what people do is done through Web Browsers and e-mail clients.
The OS is always going to have some relevance to the world, especially in the business world, where apps have to be created to run on the OS. Windows has different API calls than Linux or Mac. Most software companies will only write a piece of software for Windows, which limits peoples choices on what OS to use. Of Course, Linux has WINE and Mac you can load Windows in a virtual machine (same on Linux). So, unless the OSes all start using the same API (Which Mac and Linux are getting closer on cause of Mac’s Unix underpinning) calls, there will always be fragmentation and less choice.
The answer is more toward a yes on the home front though. Most home users are basically doing e-mail (Webmail can be used), Surfing the Web, and doing Word Processing (There are web things such as Google’s word processor). Course the gamers will have to watch out on what OS they use, but overall, all of the above items can be done on any OS rather easily.
So the real answer is that the OS choices will still be around, but for those who don’t need a lot, you can get a free OS that will do what you want. For the corporate world though, the OS still does matter, at least until all the apps become web based. Then all bets will be off.