Interesting review
I've had a GH1 for nearly a year now, and have been very impressed by it. I'm by no means a skilled photographer, but have found the intricacies of the Panasonic easy to get to grips with.
The GH1 is, however, significantly more expensive than the G2. The kit lens is a big part of that, with the GH1 coming with a superior 14-140 (28-280 equiv.) lens (with an actual OIS button!) that's worth about 600 quid on its own. The only other major difference externally is a stereo mic (though placed so close together, they have little effect). There's no touch screen on the GH1 either -- sounds like it might be handy for video, but a bit unnecessary for stills (really, who uses the screen to compose stills?). Other than that, some of the controls seem to have been moved around (I like the iA placement, the jog dial and video record, less so).
In terms of the internals, the GH1 can record video in the regular AVCHD format, meaning 720p at 50/60 fps, and 1080p at 25/30 fps. It would be interesting to know if there are any actual differences to the processing chips and other gubbins, or are the restrictions on video simply down to firmware. I ask, as some enterprising people have hacked the firmware of the GH1 so that you can a) use 3rd party batteries, and b) ramp up the AVCHD bit rate from 17Mbps to waaaay higher. Apparently the video you can squeeze out of it is now absolutely stunning. You'll need a pricey SD card though (I have a Class 10 8MB card -- great for video and shooting in JPEG+RAW). So, what chance that these guys could hack the G2 firmware to similar effect? Could make this camera a bargain!
Re TTL and flash -- Panasonic flashes are just rebadged Olympus models (and oddly more expensive). I bought a pretty powerful (and slightly oversized compared to the camera) Metz which supports Panasonic TTL. Seems to work fine, particularly when bouncing the flash.
All in all, the GH1 is a great camera, though maybe a little overpriced these days (I bought mine pre-EOS 550D). It looks like you can get 80% of the benefits of the GH1 for a little over half the price with the G2, with a lens far better at video than the Canon kit lens on the 550D.
I'm now very interested to see what Panny come up with for the GH2.