I was on a safari in Kenya a couple of months ago with a GH2 and a G3, with various lenses which I changed relatively frequently. No dust problems, while friends with Nikons (D700, D7000 and D3s) next to me did have problems.
I used the 100-300 most of the time, but the 14-140 did get quite a bit of use, and the 7-14 as well. Fixed focal lengths were used only in low light situations.
The 100-300 on the GH2 was the best combination, but the G3 worked fine as well. Either needs some setting up as the options are rather extensive. I had about 6 batteries for each camera, and used up to 5 on the GH2 in a day. Probably about 2000 shots on a day like that. 4 of each kind were third party batteries, and they worked fine, but the OEM batteries had greater capacity. No problem, as I always had extras with me.
I went to South Africa in 2000, and was in a private reserve for a while. The vehicles there made anything except handholding a clumsy proposition, unless one was the only person on a bench. Even so, handholding works better. At that time I shot with Canon EOS3 and the 100-400 mostly, often with the 1.4x. This worked, but certainly did not deliver the quality of the GH2 or G3 with the 100-300, in large part because of the superior optical quality of the 100-300 vs. the 100-400 Canon.
The typical 'pop-top' Kenyan vehicles allowed you to brace your elbows on the ledge of the roof frame if you were taller, or use a bean bag if shorter but handholding was still the best option with m43 and the 100-300.
I mostly limited my sensitivity range to 800 or less and that was sufficient. As the sun dropped precipitously, there were only a couple of minutes where higher ISO's would have made a difference. In South Africa the transition time is somewhat longer.
Image quality is essentially identical between the GH2 and G3. Controls are better on the GH2, battery life is slightly better and that is really the sum of the differences IMHO.
Here:
http://gallery.leica-users.org/v/hwulff/trips/kenya/P1090302.jpg.html
and the other pictures are example shot with the Panasonics in September.
I did not bring my Leica M9, and do not have an MF camera to take, but am quite satisfied that I took the right gear, considering that I'm unwilling to buy/carry big gun DSLR lenses and ancillary equipment.
Henning