I was in Heathrow before and Bob Geldof walked past and people actually stood up and clapped as he did!!
I remember reading an interview with Kate Ritchie from Home and Away a few years back and she said one Christmas she just wanted to get out of Sydney and have a normal Christmas where nobody would know her. She said she settled on New York where she wasn't known, booked a restaurant for Xmas day and when she walked in a group of Irish recognized her and started plaguing her.
I think generally it depends on who you are a where you are, a H&A star is bound to be recognized by a group of Irish in America but I've seen Eastenders walking through central London and haven't batted an eyelid