Fallout: New Vegas patch 1.5 - released on April 25, 2011. Fixes numerous issues. Available in Europe, despite PSN being down when this patch was released it was still available to those connected to the internet.
Fallout: New Vegas patch 126.96.36.1994 - released April 28, 2011. Various weapon adjustments. A second version of this patch was released on April 30, 2011. This second version was issued to get rid of a bug that caused certain users to be unable to load their saved games. Added Zion achievements.
From the XMB navigate to Game → Game Data Utility → Fallout: New Vegas → (X) → Delete → Yes. The total size is ~4GB this is because this is all the game data that was installed when you first played the game. The patch downloaded from PSN was integrated within this data. You must delete all this data to remove the patch. The game will then reinstall itself next time you load it up. This will not delete game saves, nor will it remove the pre-ordered content, as they are separate file.
Note: after removing the game data and installing it again: When you attempt to load from a game save, the game will warn you that you are missing content. Select yes on the prompt to be able to continue, as this is just referring to missing updates.
DLC Note: If you remove all the patches from the PlayStation 3 version, and play the DLC some bugs will occur such as Dog not being able to talk, therefore halting the DLC and it's progress. Also Honest Hearts will not show up as a quest.
From the Xbox 360 Dashboard, go to the 'system settings' and select 'memory'. From there highlight your Xbox's primary storage device, and press Y. From the next menu that appears, select 'clear system cache'. Select yes on the warning screen that appears, and you're all done!
Update: The Xbox 360 process has changed for later updates. To delete the patch, go to system setting → memory → primary storage device → games → Fallout: New Vegas, and delete "Fallout: New Vegas title update #4". When you start up the game again, you will be told that there is an update to the game. If you opt not to download it, you will be signed out of Xbox Live. Upon loading a saved games, an error message will appear stating that the game relies on content that is no longer available, with an option to back out of the saved game load.