Falsifiability, and definitions...
So the hypothesis of the nature of samsara, and the hypothesis that a person can break free of the wheel of samsara and reach Nirvana, are testable, repeatable, and falsifiable? And to do that, of course, Nirvana must be well-defined.
This is a really good question. The answer is, of course, difficult. My father will probably accuse me of sophistry, but here goes anyway:
Imagine for a moment that you are an old-time person, who has never been more than a mile from the place where you were born. You have never in your entire life met a traveler passing through - you only know local people, all of whom speak the same language. One day, a traveler comes through. This traveler speaks your language, but explains to you that it is not the language he speaks at home. He demonstrates by uttering a stream of gibberish.
You don't believe him. You have never heard of someone speaking a different language, and it doesn't make sense. This person is making a wacky claim, but offers no falsifiable proof, unless you are willing to walk with him, a journey of many weeks, to get to the place from whence he comes.
Is he lying to you? Would you walk with him? If so, why? Imagine that you are the traveler: how would you prove your point to this person?
In fact, this stranger could probably logically convince you of the possibility that there might be more than one language, if you were willing to listen. But you'd have to be willing to listen, and in order to follow his logic you'd have to be reasonably well motivated, because while the existence of foreign languages is obvious to those of us who live in these times, it would not be obvious to this hypothetical person from the somewhat distant past, and so the proof would of necessity be fairly detailed.
Now, what if the claim were about something having to do with your own mind. I could come up to you and say "Yo, dude, I am an enlightened being. Bow before me, and make offerings to me, because I totally rock." And you would rightly say "Fuck off, Ted! You're a regular dolt like me, only, based on what you just said, clearly not very bright." This is because I am making a claim about a state of mind, and such claims can only be investigated and falsified by someone who is able to perceive the state of the mind on which the experiment is being done. That is to say, the person whose mind it is.
So one way to approach this is to say "okay, this dolt can't prove to me that it's possible to reach the state of mind that he claims to have reached, so clearly he's an idiot, and I will ignore him." This is a perfectly reasonable response. People make lots of assertions about stuff, and usually they're completely full of it.
Another way to approach it is to say "wow, this guy says he's enlightened. I will bow to him and worship him, and maybe he'll be nice to me." This is actually a really unhelpful response. Who cares if he got enlightened? The real question is, can I?
A third approach is to observe that the claim this person is making is neither obviously true, nor obviously false. At this point, you can either say "look, I don't care about this enlightenment thing, I'm not wasting any effort on this," or "this enlightenment thing is interesting enough that even though I have no way at the moment to verify that it is possible, I want to test this guy's claims."
If you choose this latter path, then because the only mind you can observe is your own, you are stuck doing the experiments on yourself. And when you're done, you can verifiably assert to another person neither that enlightenment is possible, nor that it is not possible, because you are the only person who can observe whatever state of mind it is that you have achieved.
Having said all that, I will point out that in fact some studies have been done as to the state of mind that serious meditators can achieve, using MRI imaging and also measuring body signs (e.g. heart rate) and electrical activity in the brain, and the preponderance of evidence suggests that meditation does in fact bring about some positive change. Whether it can be used to reach the matchless state of a totally enlightened Buddha is neither supported nor not supported by what evidence is currently available, nor can I see any way that if such a state were possible, you could measure it using these techniques. I don't know enough about these studies to know how carefully they have been done - to be frank, I'm not that interested, because I don't need an MRI to see what my own mind is doing.
So this pursuit will probably always remain the domain of those who are willing to do experiments on their own mind, and have sufficient motivation to actually carry out these experiments, even though they require a great deal of long-term effort to perform. My friend on the mailing list is mistaken in thinking that there is no definition of nirvana - there is, and it's a very careful definition intended to precisely describe the relevant qualities of the state of mind one is attempting to reach. However, he is also right to question whether it makes sense for me to assert that such a state in fact exists, and if so, in what context.