Not in my experience. It might be more accurate at a higher speed, but my Garmin Nuvi can sense me moving it around at arms length while standing still. The speed and direction are smoothed out over a couple of seconds so it pretty much reflects what's going on unless you're dodging alligators or something. I've used the pedestrian mode on the Nuvi once and it looked OK to me - it certainly told me I was walking down the street in the right direction, and it showed when I turned down the wrong street (just testing ).

EDIT: I guess you're reading this article. I will mention that the guidance I received in pedestrian mode was specific to a pedestrian - it directed me down the Atlantic City boardwalk, which I think it would not have done if I was in automobile mode (at least I hope not).