Well there is finally more info coming out about Fable, which is due to release in the US in September and eveywhere else in October. These articles go mainly into the trials and tribulations of attempting to be a good character and an evil character. Thankfully this game looks to be incredibly deep and maintain a purely good or evil stance will be very difficult or nearly impossible; at least it will be more difficult and complex than KOTOR.
There was also a tidbit about how when the main story is completed and the credits role... THE GAME IS NOT OVER. Your character and the living world around you continue with new quests popping up now and then and it lets you do whatever you wish to this world you helped create.
About being good
http://xbox.gamespy.com/xbox/fable/528077p1.html
About being evil
http://xbox.ign.com/articles/527/527974p1.html