The Google Nexus One managed to stay up-to-date all this time mainly because Mountain View managed to keep its promise and continuously push out refreshes as they were stable. The phone recently got its Android 2.3.6 update.
The Nexus One Android 2.3.6 build is GRK39F and, from the description, it contains “important bug fixes and security patches”. What you need to do in order to apply is to download the ROM file from the source link and then go through the update process (you most probably know by now). Let us know if you find anything out of the ordinary in the new ROM.