I am happy to jump to conclusions and I shall continue to do so.
If you're saying that the reason you've installed the OEM version on 3 motherboards is because each of the first two failed and on each occasion you were sent a warranty replacement that happened to be a different model then you are indeed still using the license correctly.
If however either of those first two motherboards failed and you used that opportunity to buy a new motherboard (replaced it not under warranty but with an upgrade) then you are illegally using the license.
I have a friend with a retail copy of XP Home.
First installed on a Sct 423 Intel p4 1.8 ghz setup, that died and was replaced by a new Sct 478 Mobo, 2.6ghz p4 CPU RAM etc and reactivated via phone with Microsoft.
The second motherboard failed about 18 months later and we replaced it with a Sct 775 P4 3.0ghz again, a different socket design, chipset - the lot.
Microsoft again,happily re-activated his license despite a complete change of chipset etc.
Surprise surprise, Motherboard #3 died and we have replaced it with an AMD 64 X2 Setup which obviously is totally different in every respect to his previous Intel based PC's - chipset, RAM, GFX Card, CPU the lot.
On activation (via the web) we were told he'd used all of his activation's and to call a number....
After a brief chat with Microsoft (and a few choice words about motherboards seeming to be pre-programmed to die after 18 months!) this copy was re-activated yet again with no problem.
The chap at Microsoft was more than aware of the system history, it sounded like he had it in front of him as he commented that he hoped we had better luck with this one compared to the previous ones!
Whilst I don't dispute for one moment that you know what your on about, my experiences (as I am the one who phoned Microsoft for my friend and built his machines over the years) tells me that Microsofts strictness with regard to licensing is far from strict at best, some would probably say lax.
The chap from Microsoft fully appreciated that , in my friends case, each time he needed to replace a faulty Motherboard technology had moved on and his previous kit was no longer available to buy (new) and therefore he had no option but to upgrade through no fault of his own.
As I'm sure you can imagine, my mate is far from unhappy with this situation, he has in the eyes of Microsoft, a legitimate operating system that is now working fine on his new PC and is a certain future customer for Vista I'd imagine as he's well pleased with Microsofts customer service.