It works worse to Alt-Tab i think. I sometimes see people do that when I'm say walking into the cube (some 2 person, some 4 person cubes) to talk to someone else. It's like, I don't know you, and even if I did, I don't give a damn if you're on the web.
A few weeks ago I was just in and on the web (nothing else open since I just got in and having coffee), my boss came right up on me (sneaky), I said "yeah, I'm not working yet", he's like "I see that", then mentioned something about the article I had open. Heh, oh well, I don't charge that time. I only do brief browsing during actual working because the testing I do has various stages that can take a few minutes each to generate, compile, run, etc, so it's that or just sit there watching it.