Honestly, I think the Kindle is just a damn good ereader. The Nook garnered some good reviews, but it was clearly outdone by the Kindle. (Go back and look at David Pogue's review of the original Nook.) The Nook Color improved on the original Nook (in most areas, at least), so it'll be interesting to see how it does.
I haven't read through every post in this thread, but from the ones I've read, it seems that a lot of people are underestimating Amazo's influence as a storefront. I didn't see the Kindle a lot on TV (and frankly, the first batch of commercials for the Kindle were attrocious, from a "What can this thing do?" standpoint), but every time you go to Amazon's front page, there's the Kindle, front and center. That garners a lot of attention right there. Barnes & Noble, on the other hand, is a bookstore, primarily. People have a lot less reason to go to that site, because the merchandise isn't as broad.
And finally, I think Amazon just got the Kindle right this time around (with the exception of not having ePub support, which I know a lot of people would like). I loved my Kindle 1, but the aesthetics were horrible. The second generation was better, but not enough to justify buying it (for me, at least). With the K3, the screen improved greatly, it got smaller, and it got Wifi. That was enough to get people like me off the fence. I think that's why you're seeing a big pickup for them.
Of course, everyone's tastes are different. But I don't see any dedicated reader currently on the market ever catching up with the Kindle, if for no other reason than having Amazon behind it gives it a big advantage.