After 911 I kind of lost the knack for street photography. I teach college, and one of the classes I teach is on urban development, so I often use the pictures I take of urban environments. I generally feel extremely uncomfortable taking pictures of people without their explicit permission. I rarely use a zoom lens, basically, I shoot everything with a 50 mm and 85 mm prime.
I am currently in Berlin, working on a little mini-project called the "Blade-runnerization" of cities (e.g., how urban environments have increasingly been taken over by advertisement and huge flashing and moving billboards).
My approach is to ask permission before I take a picture, especially when taking pictures of official buildings (goverment, etc.). If I see a security guard, I go up to him and ask whether it's ok to snap away (rolling out my credentials if necessary).
I used to love taking pics of kids. But now? Forget about it! Luckily, I have 7 step sisters and brothers, so there are plenty of photo opportunities.
I also think it's important to respect people's private spheres (independent of what the law allows you to do or not), so clandestine photography is out of the question for me.