For my own reference and because I couldn't find anywhere else where this had been documented. Here are the different OS Version numbers for each of the WP7 releases/updates.
7.0.7004.0 - Original public release
7.0.7008.0 - "Pre-Nodo" update (contained changes to support the NoDo update)
7.0.7390.0 - NoDo (No-Donuts) update (included performance improvements and cut and paste)
7.0.7392.0 - Security update
Update: See http://www.microsoft.com/windowsphone/en-us/howto/wp7/basics/update-history.aspx