Nookie Froyo, the Nook Color's Android 2.2-based custom ROM, can now be flashed via ClockworkMod Recovery.  Developer modembug (not the developer of the ROM) has managed to bring us a working .zip of the current Nookie (0.5.9).

You can check out the detailed instructions here, then browse around in our Nook Color forums to learn about overclocking Nookie and more.  [xda-developers]