Nexus one got OTA update FRG83 which updates the Android Version to 2.2.1. The OTA update is 3.9mb in size. The OTA update comes with important bug fixes and security patches. The notable security fix is Google have disabled the one click root in Nexus One. The OTA update is having issues with swype and other third party keyboards. This update is only for those running the latest version of Android 2.2 (build FRF91).
Goto Settings>About Phone, and select System Updates to check whether the update is available for your Nexus one too.
P.S: You will lose the Root when you install this OTA update.