So no.... i don't think the motherboard would be the "key". it probably follows generally the same rules as the retail version.
http://www.pcmag.com/article2/0,2704,2087792,00.asp
"It's important that people understand the OEM EULA when it says that your OEM Vista (XP is the same) can not be transferred to a new computer, and a new (upgraded) motherboard is also considered a new computer," the system builder added. "I've activated hundreds of PCs over the years and have never had an activation rep ask if I am transferring the OEM software to a new PC. The only question that you must answer correctly (no) in order to receive the activation code is: "Is this version of Windows on more than one PC?". Therefore the end user assumes that since they received an activation code they must be legal."
So the short version of the EULA is that you can't [legally] transfer to a new MB, but everything else is fair game.
I don't reinstall Windows often, but I did have to call them once and it seriously took 3 minutes. IIRC I had to voice in my product key and a machine read back to me an activation code.
BTW, I am not a Vista basher, I didn't have any preconceived notions about it, but I actually used it, for months, and can honestly say my eXPerience was horrible. I originally had it installed on a P4 2.8 with 1GB of RAM and a GeForce 7300 512MB, it ran noticeable slowly. That is not a new computer, but it sure can run Linux with full 3D desktop effects without issues and XP quite well. I then installed it on a X2 5200+ with 4GB of RAM and a GeForce 7950 512MB, that ran a lot better but it was still very noticeable slower than the XP installed on that. Both PC's have multiple HDD and I experienced the molasses like transfers for even small files between them. The good, the UI is slightly polished over XP. The bad, lets see, my scanner doesn't work, many programs I use don't work [expensive ones like ACAD 2006], UAC is extremely annoying.
The reason I finally deleted my install was that it decided one day that my network should be "local only". After searching Google and the MSKB, basically told me to reinstall my drivers. I did that, didn't work. Went ahead and removed my LAN from Device Manager, rebooted and let Vista "find new hardware" and reinstall the drivers, that didn't work. Basically did all of that over again and magically my network was back. Then a week later it happened again, I deleted the install right then. Its not worth it. You make it sound like since I [and many others] have a negative opinion of Vista it is somehow invalid.