Tom,
This just doesn't sound right. My friend has a new G5 and my Panasonic works with iM5 just fine on his computer. I'm sure you've got the correct Firewire cable, but just to make sure, yours should have the small square 4-pin on one side and the largerer 6pin on the other. Just to be safe, make sure you have the cable plugged into Firewire ports and jiggle the connections while the blue screen is showing. Make sure the camera is playing when you do this so if you do get connection, you will see something. Did your camera come with software? It shouldn't matter, but just checking. Sometimes software helps the camera identify. Next, with the camera turned on and Firewire cable plugged in, try restarting your computer. Sometimes the computer needs to reset how it sees the ports. Don't be too mad at Panasonic for not having any info on their site for Macs. That's the way it is with most webistes. If you wander over to the camcorder discussion board on CNET.com, it's filled with people who use PCs and who can't download footage into their computers for editing. Its kind of funny. It's also not the way with Macs. I guess there could simply be an imcompatiblity issue with your camera. I've heard that some Samsung and Canon cameras have issues. I just don't know any obvious things that I can point you to. Try doing a search on this iMovie message board for someting like "Firewire not working" or "Camera not recognized" and see what you come up with.
Oh, one other thing. Isn't the NV-GS15 a PAL camera? I can't tell from your profile if you are in the U.S. or in a country that uses PAL. If you are in the U.S., that may explain your problem. You can play a PAL DVD on a Mac in the U.S. but most are configured for NTSC. If you are in a PAL country, is your Mac PAL?
Dan