M. Gilden
Canon DSLR Moderator
The nikon D5100 and the GH2 seem like good options but are just a bit big to carry around at all times.
The GH2 is the slightest bit bigger than the other ones you have mentioned, but no where near the size of the Nikon D5100 IMO- if that's what is stopping you, you may want to try actually holding one. With a pancake prime attached, it totally fits in my coat pocket which is more than I can say for any mirrored DSLR. Here's my GH2 with the 17mm pancake next to a T2i with the popular Tamron 17-50mm zoom. The Nikon D5100 is about the same size as the Rebel series, so this will give you an idea of the difference in size. I also threw a Pentax point and shoot in the background for scale reference.

While I was at it, I quickly threw my Olympus E-PL3 with its kit lens (since the prime I usually keep on it is currently still on the GH2) for reference. And the E-PL series is about as compact an interchangeable lens camera as you'll find these days. It is smaller than the GH2, but the GH2 packs more controls as well as an EVF and flash bulb.

Will the GH2 fit in a jeans pocket? No, only the point-and-shoots with a retractable lens will. But these mirrorless with thin primes will easily fit in my coat pockets to carry around if I wanted. Not the same class as the Nikon or Canon options at all!
I think it has failed to attract the attention of the majority of people looking to shoot movies. Could be because the mount is yet another "standard" which will require adapters to use most glass, could be because the video quality and resolution is still "canon-esque", and for the same amount of money (or less) you can get something like the GH or NEX series, which packs more image quality into roughly the sane size body, and has more options in native glass as well as a plethora of cheap mass-produced adapters.What really caught my attention is the Canon M tho - actually I'm surprised it didn't come up before. Any thoughts on that one?
That doesn't mean the EOS M isn't good for video- it may be. It just isn't as popular a choice as one would think.