Android 2.2 Froyo had been officially made available for the Nexus One along with an option where users could download and install the manual update on the Nexus One. However, if you are not that tech savvy you can easily root Android 2.2 Froyo on the Nexus One using a tool called Quick Rooter.
According to an XDA forum member, Quick Rooter does not install a full ROM, however, it should be used to flash an existing installation.
Just a quickie to enable root access on the new FroYo OTA.. This is *not* a full ROM, but meant to be flashed over an existing stock installation. This is just something I threw together for myself until Google releases the source to AOSP and CM6 can get underway.
If you are eager to root your Nexus One and get Android 2.2 Froyo on your device, visit this page on the XDA forum to find the necessary downloads to flash your device.