The Feb., 2009 issue CQ magazine had an article talking about "hidden" antennas and operating in restricted housing developments and included the following info on testing done on induction distances for HF interference for speakers, stereos, TV surround/sound, touch on lamps.
100 w              20 - 25 ft
400w               200 ft
800 - 1000 w    500 ft 
 
The following is from the RF safety tables in the General Exam study book
 for 500 w radiated from a dipole,  minimum allowed distances in meters from radiating element.
 1 meter = 3 ft, 3 in. 
                        controlled          uncontrolled
80M                     0.6                    1.3
40M                     1.1                    2.5
20M                     2.2                    4.8
10M                     4.5                  10.0