This was originally going to be part of my mod Fallout: Unity, but I figured it was best to separate it out for people who wanted the functionality I'm adding without this feature.
What I have done is taken the vast majority of Fallout 3's dialog and changed the players prompts to sound more like the Courier. The difference in player tone in the two games was extremely jarring to me. So, I went through the game quest by quest and changed what I felt needed changing. This doesn't make you sound like a hardened wastelander coming out of the vault - what it does do is change most of the dialog to neutral voice, which makes at least one of the dialog options sound like something you would conceivably say. See the attached screenshots for examples.
If you find any bugs with the dialog - typos, repeated lines, you didn't feel a particular line's tone worked - take a screenshot of the offending interaction and post your thoughts.
This changes none of the speech challenges, as that is a job TTW will eventually do itself. I highly recommend http://www.taleoftwowastelands.com/content/new-vegas-style-speech-checks-fallout-3-content as a companion to this.
12/12/2013: Fixed typos in Take It Back! dialog with Autumn and The American Dream with Lyons.
12/07/2013: Fixes to dialog with Reilly and Eden. Also added screenshots.