Who knows? One day you may have a Mac that can triple boot into either Win XP, Mac OS X, or Linux. With a setup like that, you can easily buy any software title off the shelf, and be able to run it right out of the box. I can see that happening eventually...
Although dual-booting is gradually becoming less important too, given the recent advances in virtualization.
Especially with dual-core CPU's, it's expected that in the near future you'll be able to run two seperate operating systems *at the same time* on the same chip.
To an extend you can already do so through VMWare and the likes, but not yet entirely native.