I have been wondering with all this single chip shenanigans going on what the lowlight capability is actually like compared to 3 chips.
Underwater this really is our biggest struggle, now with a 3 chip cam one chip will pull the red signal but if underwater we lack info in the red channel do we really only work on 2 and a bit chips???
Is sensitivity directly related to surface area?
I reckon this is a post for Drew to pull apart but I'm interested in if really a single chip cam for example the new Panny Af100 will be a lot better than the Ex1R etc.
The simplistic answer is YES, sensitivity is related to sensor surface area (and pixel size)...but what you gain on the swings, you potentially lose on the roundabout.
After fooling around with the Canon 5D2 (BIG chip) I've now happily settled with GH13's (M4/3). The Canon had excellent low light capability...but was a pain in terms of focusing underwater because of very narrow DOF. Not to mention moire and other issues due to the big sensor being downscaled to 1080p video.
The smaller M4/3 sensor is a good all-round balance IMHO. Excellent low light performance at a given ISO (I never use greater than 400) relative to small chip camcorders I've used. Of course several different parameters come together in any camera design to provide the final video IQ (eg, chip size, codec, bitrate, degree of skew, GOP length...yes you can even tweek GOP on the hacked GH1... optics, etc). The image quality out of a well tuned GH13 shooting H264 @ 60mbps (or MJPEG @ 100mbps 4:2:2) is simply better than anything I've seen this side of a RED ONE or a Sony F900. The lenses are quite superb...but you can also put very fast prime optics on as well to improve low light acquisition.
The Panasonic AF100 should be better in some ways (better codec impementation; better CMOS design...especially in low light)...but worse in others (AVCHD @24mbps only). However, the AF100 outputting SDI to a Nanoflash should be very impressive. Then there's the new GH2...not hacked yet, but it might just allow clean output via HDMI to Nanoflash type devices.
The short answer to your question...I could never go back to shooting with a small chip camcorder again. I just spent several weeks in Indonesia shooting with a couple of GH13's and am very pleased with their performance and the results.
For topside examples of low light work with a GH13 have a look at:
http://www.vimeo.com/16233233
http://www.vimeo.com/17130545