One of the developers with the open source Android firmware project tweeted the news yesterday. According to the project's stats page there are now 504, 372 Android devices running CM7, with not quite 4 thousand new ones since yesterday.
CyanogenMod is a customized aftermarket firmware for Android devices (around a couple dozen are now supported, including the NookColor).The current version is based on Android 2.3.3 and it includes a lot of features not found in some (most?) official firmwares. It's intended to increase performance and reliability over the stock firmwares released by vendors and carriers such as Google, T-Mobile, HTC, etc.