I got my nook last Friday and now I had some time to root it ;-)
Thanx to the people at there was not much to do.
They have a very good howto in their wiki at http://nookdevs.com/Softroot that I used.

I hope I can return something to the community with a cool app for the nook but I need an idea...
Please comment to this post if you have an app idea that could be useful for the nook.

Download mirrors

I mirrored the two update files on uploaded.to because it took me a long time to download from the original mirrors:
signed_bravo_update.1.0.0.dat => http://ul.to/fopfnt
bravo_update.dat => http://ul.to/mbvktv

Downgrading the nook to v.1.0.0

I charged my nook almost fully and the just connected it to my laptop with the USB cable, copied the signed_bravo_update.1.0.0.dat over to the nooks internal drive (Drive name "nook", file directly in root of partition) and renamed it to "signed_bravo_update.dat".
After this I unmounted the nook drive and disconnected the USB cable from my nook.
It starts with updating as described on the nookDevs wiki and continues for some time, rebooting a few times and you can see the progress on the small touchscreen.

Upgrading to customized v.1.2

As preparation I formated a 1 GB microSD card to fat32 (labeled as "nookext") and inserted it to the nooks extra storage space behind the back cover.
After connecting it again to the laptop (after downgrading) I had two drives, the nook and the nookext drive.
I copied the bravo_update.dat over to the root of the nookext partition, unmounted both drives and disconnected again the USB cable from my nook.
Turned off the nook with a long press on the power button and started the update like said on the wiki:

1) Press and hold the upper page-flip button on the right-hand side of your nook (the one marked with a < pointing towards the middle of the e-ink screen).
2) While continuing to hold the page-flip button, press and release sleep/power button on top. Don't let go of the page-flip button.
3) Continue holding the page-flip button until the e-ink screen displays a "checking for update" message. Release the page-flip button within a couple seconds of when you see this message.
Timing is key. If your nook displays the typical "Starting Up" screen, you've missed it—wait until it starts up, then turn it off and try again. 
Text copy from http://nookdevs.com/Softroot

I did not see the "checking for update" message but it worked on the first try...

All done now, I can enjoy my rooted nook ;-)