Ah... this works a LOT better, too:

gst-launch v4l2src ! ffmpegcolorspace ! pngenc ! filesink location=test.png

That'll get you the full-sized image in png format. Note that there is a delay after issuing the command of several seconds, which I can't seem to track down.

Indeed autofocus and flash are a bit tricky... you should be able to use the camerabin device, but its expecting to behave like the camera itself, so it doesn't play too well with gst-launch