Tonight's mostly failures at the heron site reminded me of Panasonic's own reticence about auto-focus on the G1. They have a default setting in a non-obvious place (not on the menu) that prevents the shutter operating at all if the camera thinks it's not in focus. So for the first three weeks I had the G1, I was getting really aggravated by the shutter not operating. After figuring that out and disabling it, the shutter operates all the time now, but with bad photos.
The settings on today's shoot were mostly f5.6 @ 1/250 to 1/320, ISO 100, and with Panasonic's very good IS, that's plenty good for sharp photos for most people, and good enough for me with other Panasonic cameras. So movement blur from handholding was not the problem. And BTW, focus on most shots, in reduced daylight about 45 minutes before sunset, took about 4 to 5 seconds. Not exactly "lightning fast".
So why won't the G1 focus? On some shots I could guess that panning a flying bird, large and slow, might be a problem for the IS at the normal setting. But the same problem occurs when not panning, so that seems to be a small factor.