The framerate on 3G/EDGE is much lower than local Wi-Fi, due to bandwidth constraints. On wi-fi, you might see 5-10 frames a second, but on 3G, you'll never see a framerate faster than 1 frame every 2 seconds. On EDGE the framerate can be even lower (1 frame every 5+ seconds). Apple required us to drastically lower the bandwidth usage on 3G/EDGE.
Please let icam run for 30 seconds or so when it's on 3G to see if the image is updating. Make sure the camera is pointing at something moving so you can tell whether the image changed.