The Xbox 360 and PC versions of Fallout 3 include 50 achievements worth a total of 1000 points, while the PlayStation 3 version includes 51 trophies (including the exclusive 51st "Platinum Trophy"). Trophies were not initially available in the PlayStation 3 version. They were added in the 1.10 patch which was released on December 15, 2008.
Every add-on except for Broken Steel adds 4 achievements (totaling 100 points) for PC and Xbox 360 or 4 trophies for PlayStation 3; Broken Steel adds 6 achievements (worth 150 points) or trophies, resulting in a total of 22 additional achievements (worth 550 points) or trophies available with all add-ons.
Xbox 360 The acquisition and notification of the achievement may, for some, be delayed up to 5-10 mins after reaching the desired level. This might be more likely if you have extra content.
Playstation 3 The trophy icons for True Mortal and Messiah have been mixed up. True Mortal shows a "Jesus-like" Vault Boy and the Messiah shows the True Mortal icon.
Achievements/Trophies are not retro-active, meaning you can only obtain quest related ones by playing from the beginning when they are initially available, i.e. "Escape" and "G.O.A.T. Whisperer." However, you can gain Trophies and Achievements by loading a previous save, doing the requirements, and then going back to your main save. This technique will also work for "branched" Achievements and Trophies like Reaver, Mercenary, and Protector respectively.
Despite both the PC and Xbox having identical achievements, the game has separate IDs for the two on LIVE which makes cross-platform profile comparison not possible. However, this allows duplicating the achievements.
PC Due to issues/overlooks during porting from GFWL, the achievement system doesn't work for GOG Galaxy and Steam versions of the game.
Xbox 360 When an Achievement is unlocked it may take several seconds to appear on screen.
The "50 Lockpick Achievement"'s File shows Vault Boy holding up a lock that has a 100 and not a 50 on it.
Playstation 3 Even though add-on trophies are included, gaining all the core game trophies grants you the platinum trophy without the need to get the add-on trophies. (This is not a bug, as this is true with Fallout: New Vegas and any other PS3 game with DLC trophies).
PC The use of console commands does not block any achievements. This allows getting all the 72 achievements with the command addachievement <1 to 72>.
The maximum amount of achievements/trophies one can get in a single play through is 64/72