How much lift comes from the fuselage on modern jets?
Not as much as potentially possible. This thread lists the numbers for some common aircraft and should give you a general idea, it seems to be ~10% 


  747: 
  
  
  Wings: 89.5%
  Fuselage: 13.1%  
  Horizontal Tail: -3.7%  
  Vertical Tail: .1%  
  Nacelles and Pylons: 1% 
  
  
  P-51D:  
  
  
  Wings: 92.9%  
  Fuselage: 7.7%  
  Horizontal Tail: -.7%  
  Vertical Tail: .1%
  


I belive the center bulge on the bottom is also there to allow for a floor recess in the cabin making it easier to get around.