Just noticed the comment on this. Yes, the XBMC forums seem to have moveable links - I don't know if their database got corrupted or what, but the link I posted doesn't seem to go to the article that was originally there. Anyway, for anyone that finds this thread, I found a better approach for dealing with overscan in the Sharp LC-42SB45U TV in a blog post, although as far as I know the method shown there only works with that one particular model of Sharp TV:
An overscan fix for the Sharp LC-42SB45U television set when connected to a computer with a Linux operating system (Ubuntu, etc.)
EDIT: Fixed link to point to new location. If it moves again, just search for the title of the page or just "An overscan fix for the Sharp LC-42SB45U".
I have noticed that many HDTV's have the dot by dot option, but each one seems to have its own name for that feature, and apparently some (like the Sharp model I have) don't present the option in all circumstances. With the Sharp, the timing of the signal makes a difference, and that can be adjusted using a proper ModeLine but figuring out the correct one to use must be a real pain. TV manufacturers really need to do a better job of documenting this, and a good start would be standardizing on a specific name for the feature so you don't have to guess what it's going to be called every time you have to deal with a different TV set.