I did a safari in Tanzania in late 2007. I used a 1ds2 and a 40D. my longest lens was a 400f2.8, and I had the 100-400 and 70-200 with me as well. I put the 40D on the 400f2.8 with a 1.4x, that gives you a 900mm f4, and I then put the 1ds2 on the 100-400. I would say my photos were about half and half, between the two cameras. You need multiple cameras. It is DUSTY. I did switch lenses some, and I would find it easier now with self cleaning cameras. Back then we'd all be cleaning sensors at camp before dinner. We started before sunrise, so some days I would go with the 1ds2 on the 400f2.8 and the 40D on the 70-200f2.8 just for the added stop of light, then switch to my longer setup once it was after sunrise.
You could go on safari with a G10 and get some printable pics. You could just take a 600mm on a crop body and get some printable pics. The problem is if you limit yourself, you will see pics and think, damn I wish I had more/less lens to take that. Multiple cameras and you're covered. The G10 reminds me, if you have a bit of light, and not too much dynamic range, a G10 or G11, etc. will cover the 28-100mm range. Don't overlook this. Sometimes it's great to have that inside the nostril shot of an animal, sometimes it's better to have a wide view of the herd, or the animal and a lot of negative space. You just have to know your equipment and what it can and can't do.
Video: I also did the POTN 3 safari this year in Kruger. Two of us had 5D2's. We would stop at some sighting, get our pictures, and then shoot video. The video is simply fantastic. My wife also shot video with a Canon HF11, and it's not bad in good light, but the 5D2 blows it away even then. In lowlight it's unbelievable what you can do. The downside is noise. A lot of my video sounds like the car motor is running even when it's not. What is running is the IS motors in the lenses. Get an external microphone to capture the sound with, maybe even an external audio recorder.
I would go 7D for the added reach, and shutter speed and it's supposed to have better autofocus. A lot of times you're shooting through limbs or brush, or maybe you're shooting a bird flying, good autofocus is nice. The 5D2 autofocus has been great for me in good light, in bad light and low contrast though, it hunts something awful. When you shoot animals you can rip off a dozen shots of a stationary animal, look through them and be amazed that only 2 are good. A blink, a sniff, ears moving to listen, all these things and more are happening on the animals face and you wind up with a lot of goofy looking pictures, a few that look nice. Maybe there's flies or gnats that you don't even see until you're culling pictures later. I had the 5D2 as my main camera this last time, 50D for somethings. The other guys had a D3 and a 1D3(with the 5d2). When a bird takes off, a leopard or lion yawns, a hippo surfaces and exhales, you'll wish you had more frames per second that the 5d2. You'll be glad you're machine gunning with the 7D.