I don't know enough about meth, but I've heard the same argument discusses on other boards. It's possible there is some property that can stimulate growth; nothing surprises me. The question, though, is finding out exactly what that property is.
You obviously can't make a generalization about meth regrowing hair, because it isn't true in every case. Still, it's an interesting phenomenon, on par with cyclosporine apparently regrowing hair. It's been theorized that the immune system is the mechanism by which hairs are miniaturized, based on the side-effects of some immune-suppressing drugs. Based on some study I read (which I can't link because the search feature of hairlosshelp.com is apparenty disabled at the moment) I don't think it's as simple as the immune system being responsible for male pattern baldness, but who knows?
I know this thread isn't about immune-suppressors, but in some ways cyclosporine's effectiveness on hair regrowth does correlate to the idea presented in this thread that meth might have some benefit to hair. It probably does have a positive effect in some way, but not in the way it might appear. I don't think we can comprehend all the different aspects of drugs unless we could isolate the different elements, and given my very narrow understanding of biology/chemistry, I have no idea if that's even possible (though anyone with knowledge on the subject, feel free to let us know).
Good post; it brings up some interesting questions.