I finally got the loan of quinophex's Magellan 315. So I went on a wander with it, to see how it faired. (The advantage of a handheld device is that it's self contained, so I can go for a walk with it easily rather than having to bring the USB device and a laptop with me.) However I'm still getting the same offsets compared to the Landsat pictures when I compare on OSM or osm-editor locally. I've set my datum to WGS84, which I understand is the default for GPS. I've made sure the GPS can see a reasonable number of satellites (5 locks is good, right? With an extra couple of marginals?). I was walking, so it's not at any great speed. And yet, as with my USB device, the track is slightly down and to the left.
I'm planning on going on a drive with both devices hooked up to the laptop so I can compare the data from each at the same point in time, but does anyone have any idea why I might be having these problems? Take a look at a screenshot of osm-editor (579KB, 1280x1024) to see what I mean; the track at the bottom is clearly shifted downwards from where the dark line of the road is.