You can do that now with Windows. What will be different with Windows 8 is you will buy it online and download it.

I was able to install the Developer Preview from USB. I have no idea if that will change.

>I expect, like Apple's Mac OS X Lion, after you install the file(s), you will be able to create a DVD disc to either install from or also boot from to setup/install the OS. In other words, less software available from companies on disc, but you can create or own setup disc burning a DVD.
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>I don't know if Windows 8 can be installed from a USB memory card, have you heard if that might be supported?