I used to do the same thing he describes. I had a core2duo in one PC and an i7 in the other. Both powered by Nvidia graphics and on 64bit OS'es.

Both Linux (arch) and Windows 7 worked without a complaint when swapping hard disk between the two. I do remember that back in the days you'd get an instant blue screen as soon as you change anything important hardware related on your rig.

The only risk factor I can see is that the hard drive might get damaged during transport.

Let's not forget that most licenses will freak out and stop working when they see new hardware. That's the Microsoft Genuine Software Advantage™

After I installed the new NVIDIA graphics driver yesterday to play Tribes Ascend, Microsoft-Security-Essentials informed me that it had ceased running because my Windows was not genuine. A second reboot seemed to placate it.

I'm curious about this as well; Windows shouldn't handle that well. Linux doesn't either but you can pretty much script it to handle it, not sure if that's possible with Windows?

I do remember now that Windows has a way to set up multiple "configurations" that you can choose at boot, but I don't recall if that will let you hop from PC to PC.