I live in the DC area and I have also seen a couple of famous people out and about. I basically leave them alone (especially if they are eating) as I think that everyone entitled to a bit of privacy.
Now one time I was on my way home from VA (traffic is so bad that I would drive thru DC to get to MD where I lived)...and I am sitting on traffic on 12th street. I am bobbing my head, because my song is on...look to the side of me...and there is President Clinton in his limo drinking a bottle of water, smiling at me! I smile back
Which in itself is odd, because in the DC area, the President is usually always moving and there are cops that drive ahead to block traffic and to make sure he is moving. But this was before 9/11 so things were different. I also saw Yasser Arfat around Pentagon City mall while out to lunch, but I wouldn't wave....there were like 2 guys each in his car and the one behind him with machine guns! So sometime its not fun to see famous people in DC
Anyway back to topic, I would probably just nod and let Catherine continue with her shopping. There is an on time IMO that she is on display and out in public working and there is off time when she should be able to be Catherine and not the Duchess of Cambridge.