ive got a zoom h2 mounted on the hotshoe for recording separately, theres a good video on vimeo of a guy reading poems in a room with different mic set ups, cant find it anymore tho, i havent looked properly but its a really interesting vid.
i found the clip i mentioned, really gave me a good idea of the difference between sounds from the GH1 with built in mic, external mic recording to the GH1 and then a ZOOM H2 recording separately.
this is only an indoor test tho, outdoors id imagine is a whole different ball game.
also found this link: http://www.dvcreators.net/shotgun-shootout/#
ps: this video isnt mine but i hope the guy who made it doesnt mind me linking it.
BTW, look up dualeyes its FREE at the moment and will sync your clips from the gh1 sound to the zoom sound, no more clapping! i run it on linux via virtualbox even.
Nice. I've used pluraleyes for mutlicam shoots and it's great. It synched 11 cameras all running wild at a concert in about 5 minutes... That's pretty darn awesome.