Some people do not stop until they find the solution. Few folks found the official link for Android 2.2 update designed for Nexus One.
If your Nexus One has not got Android 2.2 update, and you can not wait longer the reap in Froyo benefits, Head over to this link and download the update. However their is the catch, it only works if you have a “non-rooted stock ERE27 system” Nexus One.
If you are on a completely stock ERD79 device (including stock recovery image), you can use the official ERE27 update (this is the same as the OTA update). Just download it, put it on the SD card named update.zip, launch recovery (power on with volume down held), press power and volume up when the warning triangle icon appears then select ‘apply update.zip’.
If you do not have a “non-rooted stock ERE27 system” Nexus One, then there is another way for you. Head over here to know more.
Update: Google has removed the update.