Using
http://download.doozan.com/nook/NookManager-0.4.1.zip with FW 1.2.1 produces a "Restart required" error after boot. After booting a second time, it said the card needs to be formatted, so I think I'll have to buy a Class 6 or higher card; the one I have is Class 4. Perhaps that's the problem.
UPDATE: It wasn't a card issue. It appears that NookManager-0.4.1.zip doesn't work with FW 1.2.1.
UPDATE #2: I get the "Restart required" error when it finishing booting, so currently my device is bricked…
LAST UPDATE!: Wow, I can't believe how stupid I was being! I dd'ed the image to the partion /dev/mmcblk0p1 instead of /dev/mmcblk0 ! (I have no idea why the card's dev name was that; that's what threw me for a loop.) No wonder the correct partition table wasn't written! Wow. Now it's working, even with FW 1.2.1. Thank God!