If you go to tableteer.nokia.com your firmware version is automatically detected and an update is suggested if there is a newer version available.

The firmwares are named in the form "OS 2007 2.2006.51-6", if you chop of the first part it does get a bit confusing

The first part is the IT platform identifier, the second number is the revision of the OS itself. The 2 here means it's the second publicly available image, 1.2006.47-20 being the first. Next is the release year and week, 2006.51 in this case, and the final number is the unique software build number.

Most devices probably have version 47-20 on them right now, while N800's assembled after week 51 will have the slightly newer firmware.