I mostly shoot with dslr's/slr cameras (Canon 1ds mk II, Canon 1v, 5d mk III with battery grip), never felt any special attention, except from the times I use my white lenses on them. (canon 70-200 f2.8 L mostly).
However, I've been on two outings with my Rolleiflex'es in the city.
- Both times I either got loooong looks/stares or had to start lecturing people about what this actually was.
The most discrete you can use now-days, is a small DSLR/SLR or a black compact, they are everywhere.