I'm kind of an idiot, but I just shot a concert with my G6, and here's what I did:
I got decent photos at ISO 100 and 200 and a tripod when possible. Noise might be a problem at 200, depending on your output size, but you can use a noise-reduction program to clean them up. 400 would give you more options, but it's unuseably noisy in my opinion.
I had best results with the spot metering in P mode, meter a couple of the brightest spots in the scene, choose one that looks good, and press * to lock metering, focus, and shoot. your photos will probably come back a little underexposed, but that's better than washed out highlights, you can usually get them looking good in photoshop. if you meter on something bright, you'll have shorter exposures so you'll also have less chance of camera shake (it happens, even with a tripod) or subject movement. Try to shoot a lot during still bits, it's hard to capture motion.
Make sure your flash and focus assist beam are off, it's totally distracting to the subjects and audience to have blue grids popping up on people's faces. Don't use auto, because the flash will fire.