(As noted by many mod authors & users, use of mods in-game can prevent achievements from completing when attained.)
The base game has 50 achievements and 1000 gamerscore points. Dead money adds 5 more achievements worth 140 gamerscore points. Honest Hearts adds another 5 achievements worth 140 gamerscore points. Old World Blues adds an additional 5 achievements worth only 125 gamerscore points. Lonesome Road adds 5 achievements again worth 125 gamerscore points. Gun Runner's Arsenal adds the last 5 achievements and 125 gamerscore points.
Steamworks achievements are subject to a number of additional security measures, set up to ensure that achievements are awarded legitimately.
First and foremost, using the console on the PC version for any reason will disable achievements for that play session. This includes non-controversial commands such as save. Simply opening the console will not disable achievements; a command must be entered and run via the console.
In either case, achievements can be re-enabled by exiting the game completely and re-loading the game. Loading a saved game in which the console has been used will not disable achievements.
If achievements are disabled when an Achievement Trigger Event occurs (such as winning your thirtieth game of Caravan), achievement progress will be reset and the player will have to redo the entire achievement (i.e. winning thirty more Caravan games). Alternatively a game save from before the trigger event can be loaded.