While the official Gingerbread 2.3.3 release continues to roll out to Nexus One phones, impatient Nexus One owners can upgrade manually without waiting for the over-the-air update.
The update procedure was described in details for the Froyo update and remains the same with the obvious exception of the update file downloaded in step 2. Make sure that you are running FRG83G build (Settings -> About phone -> Build Number), download the FRG83G-to-GRI40 update file and follow the rest of the procedure.
Important: before performing an update, remember to charge the phone. It is also a good idea to backup your bookmarks, sms and call history, etc. using one of many backup apps such as My Backup Pro. Titanium Backup is another extremely powerful backup app, but requires a root access. Both free and purchased apps and contacts will be restored from the cloud automatically. If you have a root access, make a nandroid backup of the system.
For Nexus One owners running non-stock ROMs, dsixda at XDA has prepared both stock and rooted update.zip files that are flashable through custom recoveries.