[Galaxy S I9000 General] xpcomputers:
My memory was that this firmware was released in UK briefly (not Nordic)... but my memory isn't perfect and I can't remember where I read that, so I could be wrong.

However, that isn't important in this case in my opinion (as you say, this would in theory only help with those in the UK or whichever country it was).

What is more important in this case, is if the hardware is failed to the point where the system won't power on, and needs a mainboard replacement..... then which firmware you have is probably irrelevant, as it is most likely they won't attempt to look. (It is admittedly a gamble though... but one I'd personally take in that scenario!)

Regarding stock firmwares from other countries other than your own, that is a greyer area, as you might still be covered, maybe someone knows better on Samsung's policy on that? Probably depends on how internationally minded they are... but my SGS is a UK unit because I'm from there, but I recently moved to NZ, so if I ever needed a warranty fix, would I have to send it back to the UK with a UK firmware on? Or could I take it here locally with a UK or NZ firmware? If I decided to put an NZ firmware on it, would it make a difference where I could get it repaired?

Beyond the Warranty, there is also consumer law in many countries, and I think if your hardware has failed, but you have an alternative firmware that can be shown to have no impact on the unit, you might still be able to get your unit fixed for free, if your firmware upgrade hasn't caused the mainboard or whatever to fail. Companies try to impose strict rules upon you to cover their backs (and give them plenty of wriggle room to escape liability!), which is fair enough where your modifications damage the unit, but where the changes you make don't affect things, then their rules don't always count. For example, many PCs come with a warranty sticker on which says "Warranty void if removed"... yet in many countries, they can't normally cancel your warranty if you've put more ram in your computer (as long as that wasn't why your machine died!). They might try to tell you that your warranty was void, but they wouldn't get far as it would normally be an unfair clause against your consumer rights. PCs are designed to be upgraded, and they can't make you use their service centre for the job as long as you didn't damage it.

Similar situation with third-party inks on a printer and them saying it might invalidate your warranty. Although admittedly that is a greyer area, as third party inks DO sometimes damage the printer. Say if a motor died, they couldn't claim your third party inks did it so your warranty is void, unless there was evidence that the inks had leaked and corroded something.

Not sure if anyone has tested this situation in practice though by speaking to consumer law experts if a warranty was ever turned down because of a custom rom. I'm sure someone who knew the law would be able to advise better where that line was. I can see that a new rom could be seen as modifying the unit for purposes which it wasn't designed for... but equally you could probably argue that the custom rom was fixing errors the manufacturer had left in which made the unit not function as intended (like GPS fixes etc), and therefore shouldn't invalidate the warranty.

If I was the OP in this case, I'd send it in and take the risk, especially since it is a stock firmware (even if it was not installed by Kies).