So eventually, after a one-day intermezzo with the D40 (which is definitely a very nice camera!) we settled on the Panasonic Lumix G1. I didn't have much time to play with it yet, but it's really quite a nice camera. It's tiny, and solidly built. It's a huge difference to the D40, which feels a lot more plastic-y. But that's somewhat expected, after all the D40 costs about half of the G1.
I've shot a few test photos indoors and it's not a bad performer there. The view finder is a bit noisy in low light but it doesn't influence usability much. Still, if it gets really dark the frame rate of the view finder decreases a lot. Not sure why, I guess the camera then averages across several frames to keep the noise down.
The auto focus is fast. I have no comparison to DSLR cameras in the same price range, but it's easily as fast as the D40 (which probably doesn't say much).
The flashlight seems a bit weak. Where the D40 tended to overexpose, the G1 is more into underexposing images shot with the flash. Might be a configuration issue, I haven't figured out all the settings yet.
I'm very pleased. I'll try to make a few outdoor shots during the weekend. Hope the weather improves over night, it just started raining again.