Last summer I installed Android 2.3 (Gingerbread) on my Nook Color. I used a technique described on the XDS Developers forum that let the Nook boot Gingerbread from a microSD card. The developer (verygreen) of this technique has apparently moved on to new challenges.
In the meanwhile, Android 4.0 (Ice Cream Sandwich - ICS) has dropped and there are nightlies for the Nook Color but the installation techniques install ICS to the eMMC, the internal flash drive. I still like the simplicity of booting from a microSD card.
The thread above has continued to get comments and questions and leapinlar has been very helpful in fielding these.
So here's how to install CyanogenMod 9 (CM9) Ice Cream Sandwich on a microSD for a Nook Color.
Use a Sandisk Class 4 8GB microSD card as recommended here. Use a microSD USB adapter and inset the microSD and USB adapter into your computer.
leapinlar has updated verygreen's microSD image to support ICS. Download the latest generic-sdcard-v1.3-ICS-large-Rev[?].zip from here. Unzip and extract generic-sdcard-v1.3-ICS-large-Rev[?].img.
If you're running Windows, download win32diskimager-RELEASE-0.3-r27-binary.zip from here. Unzip and extract the folder win32diskimager-RELEASE-0.3-r27-binary. Run Win32DiskImager.exe and write leapinlar's image (generic-sdcard-v1.3-ICS-large-Rev[?].img) to the microSD card.
After Win32DiskImager is done with writing, eject the microSD and USB adapter and re-insert it into your computer.
Download a CM9 nightly build from here. Pick the newest that does not say opengl. Be sure to scroll down to the bottom of that page to get the newest build.
Download the Google apps for Ice Cream Sandwich (currently gapps-ics-20120429-signed.zip) from here.
With the newly imaged microSD and USB adapter back in your computer, copy the CM9 nightly build zip and Google apps zip into the root partition (the only one you can see) of the microSD card.
Eject the microSD and USB adapter from your computer.
Make sure that the Nook Color is powered down and insert the new microSD card into the Nook. Power it up.
If the Nook doesn't boot at this point, revisit the microSD and USB adapter and the downloaded .img file. Explore alternatives perhaps using a different microSD and/or USB adapter and redownloading the .img file.
Sit back and watch. It will take a few minutes and lots of messages will scroll by. Then the screen will go black. Power up the Nook.
Ice Cream Sandwich!
I still had to use the Google Play Store to install Gmail and youtube.
See also the errata.