+1 for GH2.
Body with kit lens (14-140) and the (100-300). You are covered for 28-600mm on two lenses and you would barely know you have it around your neck. All in for about $2K. Oh, and the opportunity bring superb video into the mix as well. I am about to head to the desert for a month with mine + a few more lenses. Having shot with 2 1/4 film for many years and a 5D for some years with all the fast and heavy glass Canon makes, I can tell you I have not been disappointed with the GH2 and really appreciate making the equipment less a focus of my time in nature, and the capturing and appreciation of that nature more the point.
Fast and long ususally = heavy. You haven't mentioned budget as a criteria so I will leave out + expensive. Under safari conditions a manageable kit can be as important as the kit itself. You likely won't want to be doing lens swaps too frequently particularly if wind is up so think hard about what and where before committing to a prime strategy. As a point and shooter, unless you know from a guide or lead photographer that what you will be shooting is a 300mm constant, man, I'd really be thinking about how many more lenses and weight I am going to need to have to be prepared and how will I manage all that 'stuff' when things start happening. Read Michaels review of the GH2 and other articles on the site to get their opinions on a GH2 on safari.
