Bethesda finally announced that the next DLC expansion for Fallout 3, Broken Steel, will release May 5 and they outlined some new gameplay details; but two things in particular stood out: you can order your companions to input the code, and Dogmeat can be reincarnated upon death.
If you don"t understand that last sentence, you should probably finish Fallout 3 first. In fact, many of Broken Steel"s features are about adding content after the main title"s endgame, so you should hold off on reading these details unless you"ve already finished the game. Anyhow, as I pointed out in an interview about Fallout 3 with lead designer Emil Pagliarulo, there are three specific characters who should be able to enter the Project Purity code instead of you, and one of Broken Steel"s changes is that you can, indeed, order any of those three characters to step in and type the code in your place.