I see a lot of people asserting statements about what you can and can't do, but little back-up (empirical or references to the EULA / case law).
AFAIK there's no LEGAL principle by which they can rightfully deny you the use of the OS you purchased on your computer SYSTEM.
If your SYSTEM needs peripheral upgrades or major repairs whatever the nature of those repairs reasonably has to be then that's fair and shouldn't interfere with your ability to use OEM Vista.
It is unrealistic to assume that you'd even be ABLE to buy an "identical" or "near identical" make/model of motherboard to your original one perhaps a year later. These things tend to come and go within 6 months for many of the not-so-prominently high end models.
Even within a given "model" number it is not uncommon for the Motherboard OEM to do major changes to the BIOS or change revisions of the chipset or other such significant changes without labeling the product distinctively for consumer awareness.
I've never seen ANY reference in the EULA to indicate any purported requirement to "repair" the system with identical model parts anyway, so unless you find differently, that's an irrelevance.
From what I've heard empirically what might happen is that it'll deactivate itself, tell you to reactivate. Online reactivation may or may not work (as is the same for retail OS versions too), so next you try phone activation.
If they ask why you're reactivating it's as simple as saying the system broke and was repaired / reinstalled, and you should have your activation with no difficulty.
Keep in mind that there's a lot of FUD about this topic and the only things you can really rely on are (a) the experiences of people who've DONE IT (reactivated or failed to), and (b) what the EULA *says*. Though the MS EULAs are notoriously unclear and it's widely known that even a lot of things they may imply are restrictions can't / aren't actually be interpreted that way in practice because things just wouldn't work and they'd get sued and get more anti-trust investigations and fines from the EU etc. etc.
Also actual law (federal / state / national) and case law precedent overrides any restrictions that may state things to the contrary in ANY EULA. The whole legal principle of "shrink wrap" EULAs is in doubt anyway and most any good lawyer will tell you that there are several OBVIOUSLY illegal or unenforcable things about most any software EULA.
Regardless of the parts making it up your computer SYSTEM is still your computer SYSTEM, and as long as you're not using your OS on N simultaneous and distinct systems AFAIK Microsoft has no reason or business (or solid legal ground) from restricting your use of the OS.
If you have to "repair" a system by changing the motherboard at least in many cases that's probably because that motherboard is a piece of junk that never worked right in some ways or wasn't reliably engineered. Do yourself a favor and get a replacement that's likely to WORK for you instead of spending good money after bad on inviting the same old failures / incompatibilities with your last one. It'd be silly to pay MORE for an identical or similar model as opposed to LESS for one that might be newer / more reliable if you do need to repair it in that way.
