"Discreet" is definitely relative.
I had my M6 with me trying to take some pictures in Mexico City recently and felt like I stuck out like a sore thumb...with all the violence in other parts of Mexico, security is very visible, and I felt like I was doing something "wrong" walking around on the streets with a camera over my shoulder.
My own answer was to go smaller and "more discreet," and I bought a Contax T3. That thing is seriously a pocket rocket. For *me*, it's basically my M6 (which I use with only a Zeiss 2/35) which I can slip into my pocket.
Recently I was at the local Science museum, and starting shooting candids. Nobody said boo...I looked like a tourist with a little P&S. Of course, I think I felt a bit less conspicuous myself, so that loosened me up.
After some internal struggling (which I'm still doing now), I've pretty much decided to take only the T3 and slide film to Paris in a week. I have the utmost trust in that Zeiss 35/2.8 Sonnar lens.